Actually, my 1st suspect is the turbo snail (are you referring to those very common snail with conical shell, sold at LFS at $1?)
Does the click sound made by the turbo snail comes from it turning its shelf left & right in a very fast manner, which I've seen once, (when it cannot get its way), but that time, there was not click sound??
For every rock that I placed in my tank (when I first set up my tank), I always do a hypersalinity dip and those bristle worms and some cocepods came crawling out so I think there's a high chance there's no mantis shrimp in my tank...but..keep my fingers crossed...