Pete: People used to do it with the Hertz Shelby/Ford Mustangs in the '60s.
Novesh: I'd sooner pull a motor out of a rental car than take the movement out of a watch. I know you're joking, but since everything has serial numbers on it, they would know its you. It may take a few months, even years. But they will get you, sooner or later. You don't think when the guy (who buys the watch after you returned it) sends it in for servcing they will call the dealer and ask where the watch has been? Then they pull up the reccords and voila, your name comes up.
Watches with swapped movements show up on eBay fairly often.
With a rental car on the the other hand, 20 people will rent the car before it goe sin for servicing, which at that point they will figure out the engine has been swapped. But since 20+ people rented the car, no way of knowing who did it.
So here's my suggestion, Hertz is once again getting some Shelby Mustangs. Anyone out there with a stock GT?? LOL