I heard a loud clicking sound in my tank, and I found a big margarita snail that was snapping it's shell against the glass to upright itself. I was worried it was an electronic gizmo making the sound...whew.
Pistol Shrimp or mantis shrimp can both be hitchhikers on live rock. You may never see them. I have a pistol shrimp in a small tank, and it dug a tunnel to live in in the substrate. The only reason I know it is there is because I can see the tunnel along the edge of the tank. Kinda like an ant farm.