About 10 years ago, the Hilton MCI royally screwed our room up, so the front desk guy gave me a stack of parking passes to use later - little yellow cards you would put in your windshield if you were leaving the car in the lot during a trip.

I did it for a few years and noticed that nobody ever asked for the cards! Also, many cars in the lot didn't have the cards in their windshield at all. I got the sense that the hotel didn't really care who parked there. The lot is completely open and unsecured...anybody can come and go anytime.

The catch is that their bus is only once every 15 minutes and circles through all three terminals. A few years back, MCI "upgraded" the regular Economy Parking so that individual buses go to each terminal, and they run every 3-5 minutes during most times of the day. So for $6/day, that's a better value to me than the Hilton for free. But I imagine that it's still an option...

Makes me wonder how many airport hotels around the world you could pull this off with. Probably not at the biggest airports where the hotels are more likely to have gated entrances or garages, but I'm guessing it'd work at a lot of medium-sized ones.