Interesting thread....I once had a similar shaila involving $$$$ that was sent to me by mistake by a major corp...I went to a well renowned rav/posek who told me unequivocally, return it!
Everything comes from above and being honest won't ever punish you...a story that is told about a major gvir that is known is when he first bought a house, they found valuable jewelry inside.
Being that they had acquired the contents, in theory, the find was theirs to keep. As a good Jew, he went to R' Moshe who said while perhaps it was yours, do the right thing and return it - Hashem will repay. The now gvir did so and as they say the rest is history.
R' Don Segal says that when we daven for parnassa on RH, we can daven that it come clean and not have to resort to anything 'shvartz'...amen.