Check at 3 am.
Creatures know when the lights are about to turn on (sunrise) and will be hiding LONG before then.
You need a late night session to really see what's going on in the tank. Recently I had a frag that kept getting knocked over and a 2:30 session revealed I had a baby urchin playing earth mover at night. NEVER see him in the day just LATE at night.
~Steve~