i used to service aquariums professionally, and i had serviced one tank for the better part of a year before i discovered their abalone, and it was about as big as a nerf football, and had come in as a hitchhiker on the live rock. They hide real well, as you have discovered. sea hares too, i had introduced one into a 60 gallon cube, and had presumed it dead after 3 months. 4 months later, i was gravel vac'ing when the tube filled with dark purple water, i stopped to investigate, and there was the sea hare stuck in my gravel vac, inking and angry, let him go, and didn't see him again for 2 months. It's truly amazing the camouflage some creatures have.