i still say mantis, if your rock is hard enough you may not hear the "clicking" i have a g.smithi hes in there pounding all the time and ive never heard him once a quick dip in fresh water or club soda would def bring him out
lots of people have used club soda before with no ill effect
you could lay a bait trap by the openings to see if anything comes out and id do it at night cause i dont think any day time critters are your problem
cheers