Ever wonder why your SIEM dashboards are telling only half the story on Microsoft 365 activity? You're not alone. The truth is, most out-of-the-box configurations miss critical M365 audit logs—leaving risky blind spots. Today, I'll show you exactly which logs Sentinel, Splunk, and others are skipping, why that matters, and how to truly close the gap.Stick around if you want your security monitoring to move beyond check-the-box compliance toward real, data-driven protection. Let’s make sure your SIEM finally sees what actually matters.Why Your SIEM Still Misses the Big PictureIf you’ve ever pulled up Sentinel or Splunk expecting to see who accessed a critical file in SharePoint, you’re probably familiar with that sinking feeling when the dashboard has nothing. It’s not just you—almost every admin I’ve talked to assumes that once they connect Microsoft 365 to their SIEM, they’re set. The checklists in the documentation say the connector is active, you get a handful of logs starting to trickle in, and it’s easy to feel like the hard part’s over. The reality? That first integration barely covers the basics, and a pile of your most important events never makes it into your SIEM at all.Let’s say you’re asked to produce a timeline of mailbox activity for a sensitive user. Or your boss wants to know who shared a confidential folder in Teams two weeks ago. The expectation is your SIEM should have this, right? Nine times out of ten, you’re left scrambling when your own dashboards come up blank. That moment when you realize you’re missing key info—especially when leadership is watching—doesn’t get less painful with experience.Here’s why this happens. Those default connectors, the ones marketed as “plug-and-play” for Microsoft 365, turn out to be a lot more limited than most people realize. Out of the box, most SIEM integrations grab a thin layer of generic activity, but miss entire categories of logs that matter most during an incident. Think about Exchange mailbox auditing—actions like “mailbox accessed by someone other than the owner” or “mail forwarding rule created” are bread-and-butter audit events for any real investigation. Yet, unless you’ve explicitly enabled mailbox auditing (and shelled out for premium licenses), those events just don’t show up.And it isn’t just email. SharePoint file access, Teams chat deletions, and especially Power Platform activity—the stuff that attackers target when they move laterally—often stay in the dark. You might see user logins or “file modified” totals, but not the details. The difference? One tells you something suspicious happened. The other gives you enough facts to actually respond.Let’s get concrete. I’ve worked with a security team that was dead certain their SIEM would help during a potential data leak investigation in Teams. Someone had shared a sensitive financial document externally. Everyone felt confident until the SIEM had nothing more than a “file shared” record, missing details like who the recipient was, whether the link required authentication, or if additional downloads occurred. Only by logging directly into the Compliance Center—separately from their SIEM—could they reconstruct any kind of useful story. That lag cost them hours and made their report look amateur. Unfortunately, it wasn’t a one-off. These kinds of gaps crop up everywhere, especially if you’re not checking connector documentation week after week.So, what actually governs which logs appear in your SIEM? A lot of it depends on Microsoft’s own auditing defaults and the version of Microsoft 365 you own. Basic audit logging, which is included with most subscriptions, captures only a slice of workload activity. Need mailbox details or sensitivity label events? Get ready to talk to finance about E5 or at least buy an advanced compliance add-on. Even then, not everything’s covered—some logs only flow via special APIs or need extra configuration. On top of that, Microsoft throttles API requests or batches logs, introducing delays or rate limits that make real-time investigation impossible at times.SIEM vendors add their own wrinkles here. Some connectors only support certain APIs or log schemas, so you’ll see Defender alerts but not granular mailbox events. Others drop categories like Power Automate runtime details, which attackers are increasingly relying on for quiet lateral movement and exfiltration. Microsoft’s own footnotes admit this if you read between the lines. I’ve run into documentation notes buried at the bottom that say things like “export of certain Exchange logs only available for E5 customers” or “SharePoint sharing events require advanced audit.” Even seasoned admins get caught off guard here—the fine print is relentless.There’s also the constant issue of API volume and throttling. Microsoft 365 generates millions of records, especially in busy organizations. SIEM connectors have to balance between pulling everything—risking cost and performance—or skipping “low-priority” logs based on size and frequency. The loser in that tradeoff? You, when you need the details after an incident.It all adds up to a messy, incomplete picture. Most organizations, even ones with mature security teams, are missing at least 30% of actionable M365 events in their SIEM—sometimes a lot more. These are the exact areas where attackers love to hide, knowing those actions are less likely to trigger alerts. It’s a weird loophole where you feel secure because your SIEM is “connected,” but the most dangerous activity still slips through.If you actually want to close those gaps, it isn’t as simple as just flipping another switch in the admin center. The questions start piling up. How much will the extra logging cost? Can your SIEM even handle the volume? Are you about to blow up your licensing budget just to see who did what in a shared mailbox? The price tag—both in licensing and in tech—starts to get real, fast. So, what does it really take to pull in the right logs and get true visibility? The real story might surprise you.The True Cost of Complete VisibilityPicture this: you finally do it. Every M365 audit log rolls into your SIEM, just like the security blogs suggest. Log for log, you’re pulling in mailbox auditing, every single SharePoint file event, Teams message edits, and enough Power Automate activity to make anyone’s eyes glaze over. You tell the security team you’ll catch anything that moves. And then—almost on cue—the finance team walks past your desk, waving a storage bill that somehow rivals your entire O365 subscription. That’s the moment plenty of security projects hit an unexpected pause. Full visibility, it turns out, isn’t free. In fact, most folks underestimate just how quickly log volume—and raw cost—spikes once you start letting everything through the front door.Here’s where things get almost comical. Most admins start their M365 SIEM journey using whatever’s included “for free”—the default audit log connector, sometimes a bit of Defender alert forwarding. You dip your toes in and see a manageable trickle of events. But that’s just surface level. The minute you need granular event details—mailbox auditing, confidential SharePoint sharing, or Data Loss Prevention (DLP) events—the magic words show up in Microsoft’s documentation: “Requires E5 or advanced compliance add-on.” It’s easy to overlook until you realize E5 licensing doubles or even triples the per-user cost for audit coverage. Even then, that’s just the M365 side of things. The minute these logs hit your SIEM, every vendor has its own take on billing. Sentinel, Splunk, QRadar—they’ll all charge for every gigabyte they ingest, and sometimes for how long you post-process or store those logs. It’s not unusual to watch SIEM costs go from a footnote to line item number one on your IT budget.Let’s talk real numbers for a minute. I worked with a midsize org—two thousand seats, mostly frontline, but a vocal finance and legal team. They’d always skipped Exchange mailbox auditing, thinking it was overkill. A new compliance push changed that. They flipped on unified audit log ingestion into Sentinel. Within a month, their Sentinel bill had doubled. They were shocked, so we dove in. Turned out, mailbox logs churned out page after page of duplicated event records—one log for the user, one for the delegate, one for every folder touched in a multi-folder mailbox view. On top of that, SharePoint events kept firing for background sync jobs, automated document saves, and compliance scans—events with about as much security value as a printer notification. When Teams and SharePoint usage spiked (annual budget season always does it), the logs came in faster than anyone could make sense of. No one had modeled out the spike in volume or factored in duplicates, so overnight, the SIEM bill was the surprise of the year. SIEM vendors are happy, but security teams often end up doing triage, figuring out how much log noise they can afford while still covering their regulatory obligations.For a lot of admins, the shock isn’t just quantity—it’s relevance. Not every log helps during an investigation, and parsing every message just introduces noise. The more logs you have, the slower queries get, and the more likely important signals drown in routine activity. Trying to chase every single Teams reply or SharePoint folder access isn’t just expensive, it’s also a recipe for alert fatigue and slow response when something actually matters.So, what do the pros do? They break down expected log volume ahead of time. Most SIEMs let you preview how much data each log type generates. You can estimate storage requirements for a typical month, then double that for periods when audits or incidents hit. Planners now start every new logging request with a data model: what categories actually yield security outcomes, and what’s just digital dust? For mailbox auditing, you might only need access by non-owners or changes to forwarding rules—those actually signal risk. With SharePoint, e

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