How to find a pistol shrimp

[weagle117]

I added a watchman goby and his pistol shrimp to a 30 gal tank a few months ago. They had been together in a 2 gal. tank for a couple of years. They swam away when I dropped them in (after acclimating, of course). The goby has been eating and hiding in the rocks but I have seen nothing of the shrimp. I assumed he died but never saw any evidence. Lately, I've been hearing an occasional click but never see him. I'm not sure it's him but sure would like to know. I've read something about putting a jar in the tank at night but I can't remember exactly what to do. I don't want to remove him from the tank though.... just find out if he's there. Any ideas?