Steve, he could be hiding in your rock too. I had my yellow watchmen in my 6 gallon nano for about 5 months. He went missing for about3 weeks, I moved everything looking for him. Assumed he died and bought another. Well as soon as second went in the first one popped out and got territorial. 6 months later I have a mated pair.

Twinspots are neat but I wouldn't keep them singly and they are supposed to be very delicate. Also they can jump like many gobies and if he did a cat or dog might have scarfed him up.