In episode three, Tracey Donnelly says her husband Jim's disappearance is illogical, and 13 years on, still believes there are answers to be found - in the Glenbrook Steel Mill.

Watch a video segment from the episode here

On Monday 21 June 2004, Jim Donnelly signed into work as usual. Thirteen years later, he still hasn't signed out.

The 43-year-old father-of-two worked at the Glenbrook Steel Mill, south of Auckland, for nearly 20 years.

His wife, Tracey, is still trying to find out what happened to him; she believes the answer lies in the mill where he was last seen alive.

"There has been no sighting of him apart from inside the mill. There has been no sighting of him leaving. There has been no sighting of him since he was seen on the gantry," she says.

"It makes me incredibly sad and upset that this wonderful person who was my husband and the father of my children disappeared without a trace."

The couple's children, Liam and Siobhan, were aged seven and five when Jim disappeared. They are now grown up.

On the day Jim went missing, Tracey called him at work about 8am and became increasingly worried when she didn't hear back. Jim had missed a 9am meeting and was nowhere to be found, yet his car was still parked at the mill carpark. He was reported missing about 6pm.

Tracey believed he may have had an accident, but the theory shifted. Search and rescue teams thought he may have had a mental breakdown and was actively evading them.

Jim had been behaving strangely in the days before his disappearance. Tracey and Jim had planned to spend the night at an Auckland hotel on the Saturday, but out of nowhere he said he had a meeting to go to.

A Saturday night meeting made no sense to Tracey, but no matter how hard she tried to find out more, Jim wouldn't tell her where he was going. He hired a suit and headed off. It's still not clear why he needed the suit.

Things between the pair didn't get any better the next day, when again Jim said he had to divert a "crisis and a waste". Tracey was confused.

Some of his co-workers described Jim's behaviour as strange on the Monday too.

Five days after Jim was last seen at work, workers at the mill found his hard hat next to an acid tank in a restricted area of the mill - the area had already been searched.

The tank was drained and some of Jim's things were found inside. The acid wasn't strong enough to do significant damage. His house keys have never been found.

Who put Jim's things there? That question remains unanswered. The hard hat wasn't tested for fingerprints.

The search for Jim was eventually scaled back…

Go to this episode on rnz.co.nz for more details