The sleeper gobies that I have had have always made dens of sorts by burrowing under rocks (usually leaving a separate entrance and exit) and then mounding rubble by the "doorways", using anything they could carry (including frags), so if you have unexplained piles of rubble anywhere in your tank, it might be your goby's den. I've never had a sleeper jump, but certainly it's a possibility. Good luck -- sleeper gobies are awesome fish.