It seems there were three very similar amps brought in (Fender Blues Juniors) and one was, we believe, mistakenly taken, probably not realizing it was not theirs.  So the one that was left evidently belonged to that person.  They all looked very similar, although the missing one has a microphone plate (for holding a mic) attached to the bottom, where it would not be noticed unless you looked at the bottom of the amp.  The owner of the missing one is an expert amp tech, and he caught the problem, and of course wants his amp back.   If you, or someone you know, might be possible owners of that amp, please let us know.  Sounds like an easy mistake to make to not notice the difference when picked up.    If you brought an amp to the jam, please check yours!  Thanks.