Oscar,
I just bought a pistol/goby pair for my "in progress" nano tank. They are very cool when in a pair.
It sounds to me like you have a pistol shrimp in there somewhere. Usually they are harmless to to most things except other shrimp, like cleaners, etc.
I would keep looking with the flashlight, but I wouldn't worry too much.
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Mark...
290g Peninsula Display, 425g total volume. Setup Jan 2013.
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