When we had danios, we had some disappear all of a sudden.
I would take every single decoration out but could not find it, then a few days later, voila! There's the danio alive and kicking swimming around like nothing happened.
This happened twice with 2 different danios.
But I caught one in the action of hiding which totally took me by surprise.
Apparently, they in search of food on the bottom of the tank, they would get themselves in the openings in the gravel. At that time we had pretty large rocks as the gravel, and since they were so big, there were gaps all through them and well, the danios would get themselves stuck underneath the gravel until they could wiggle themselves out.
I swear about give me a freaking heartattack.