I think a third of my posts are about the livestock eaten by my Engineer goby. The day I finally caught mine was one of the best that I have had in the hobby. It starts with a suspicion. Then you are sure but you have no proof. The next thing you know you are down 4 or 5 small fish and the thing has a big old gut. If it gives you the opportunity, I'd get it out of there.
I don't believe that they are all bad. But Mine sure was.