what you SHOULD do
lots of smart people responding, but only one had advice on the appropriate response to finding an unknown-sourced USB -- and that was to put it in somebody else's computer -- an ethically questionable move.
So WHAT should you do? I would assume either
a) there is somebody who legitimately is missing their USB, so what should I do to be safe?
b) some spy person is doing something BAD, so ???
c) ?
Is there some existing security harness that I could buy/build and use?
Is there a set of "safe" procedures for accessing without compromising the host system?