Lost Sea Cucumber

[tyrant]

I put a sea cucumber in my tank about three days ago, and now he is no where to be found. I have approximately 3" deep worth of sand on my top layer that he could have possibly burrowed into. Should I be worried about him, or is this normal.

 

[tyrant]

I did finally find him. He was about midway up the height of the tank in the rocks, very well hidden. I assume that they are nocturnal creatures, since I never see him in daylight. I can only see him at night with a small flashlight.