Put a red light on your head and do some late night adventuring. I'd wait a couple hours after lights are out and make sure that all the lights in the room are off and sit around to see what you find. This happened to me when I first got into the hobby and I found a large hairy crab. Watching the tank with a red light was creepy...reminded me of snorkeling my favorite surf spots. You are better off not knowing what's there...lol!