You're in the coffee shop, you hear some idiot (Joey) ask if anyone has lost their keys.
Later, you see Joey polishing a Porsche. You put 2 and 2 together.
You go back later, walk up to Joey cleaning the car and say, 'hey, that's my car.'
Joey then hands you the keys and you just made a huge profit.
That is a good point actually, I've never thought of that. It could have actually made a really funny ending scene if a after the "owner" came and took the car, Joey was standing where it was parked, looking disappointed he can't pretend it's his anymore, and then another guy walks up in a panic and says,
"Has anyone seen my Porsche, it was parked right here!!".
Joey: "Yeah don't worry, the owner came by a little while ago and I gave him the...."
And then he does his massive eye opening thing he does when he's shocked.