Keep it and wait for somebody to report it missing. If somebody came forward, I would return it to them. If no one came forward to reported it missing, I would assume the reason they did not report it would have to do with dirty money and I would KEEP IT. I would hand money to the needy at my local trailer park. I would bankroll some Porno movies; producer director and smoke crack with my actresses.
It would depend if I know whose it is? If its just cash then my biggest problem would be me explaining to my wife that those lumps in our new 2k mattress are suppose to be there!! LOL
I would head out to the watering hole and buy drinks all night long. I'd get plastered. Then I would buy a few friends cars and stuff they need. I would leave c note tips all over the place. I'd call an escort service and have them send up every girl they got working. I'd take the girl I could never have and take her for two weeks to Vegas and tell her I got a lot more where that came from. Buy me a Vette and Harley cash. Go a few nights with the local, betting multi dimes. Tell the local unemployment office to shove it and throw the remaing food stamps back at the caseworker.
And I would be very careful not to raise any suspicions in case somebody was looking for a dude who might have found a million.
Well being that I live in Vegas I would bring the suitcase home empty it out put some losing betting tickets in the suitcase return it to where I found it with a note saying "Sorry about that!"