Offer a $2,000,000 prize for the return of the key with no strings attached.
Then when they go to return the key, plant drugs in/on their car while distracting them/when they come in to return the key. Then use a sniffer dog reading signals from the handler to "say the car is hot". Search car and arrest them on an intent to distribute charge and then cite civil forfeiture to take back the $2,000,000 and done. First round's on me guys.