I used to breed them (
Elacatinus oceanops) at an aquaculture facility I worked at - there was a pair in a 15 gallon tank with only one small rock in it, and a couple small PVC tubes stuck in the sand. They would go "missing" for days at a time, not to be seen at all, even when searched high and low for, only to re-appear in the next week or so.
They're amazing little hiders