When children become quiet, they are into something. Usually, it's mischief, just to be honest. Not all the time, but it's been my experience that when kids become quiet, they are most likely sticking a fork in a wall outlet, pouring chocolate syrup on the floor or having other grand adventures.

Even we older folks get this way. We get the same way when we are into something, or something is bothering us.

Check on your people.