Ethics & Practicality
Ethically, it's a no-brainer.
You call it in, wait for the guys in blue, report the event. Additionally, I wouldn't want something a bad guy's been using unless the PD gave it a clean bill of health and made it available.
Practically, it's a no brainer.
ANYTHING the bad guy dropped -- gun, drugs, bag of cash, jewels, sports car -- anything at all, you turn over as evidence.
You don't want anything in your possession that could be tied to a crime. Once more, if the PD vets it and declares it clean, and should they offer it as "unclaimed," then sure, take it. But not until it's been through quarantine.
I'm not familiar with police procedure and the rules and regs surrounding "found" goodies of this sort, but I wouldn't hold out a lot of hope of getting a phone call to "come collect yer booty."