Thus, a question for all of you. What would you do? Would you return it, or would you keep it?
Edit - oops, this has been said, "Integrity is doing the right thing even when no one is watching." Your answer tells what kind of man you are.
It's definitely not cut & dry, but for me personally, I don't buy the its-OK-to-screw-institutions-if-theyre-big-enough line of reasoning.
What part is not cut and dry? Something showed up at your door that you didn't pay for and doesn't belong to you. What ethical standard would you reference that would cast doubt on what you should do?