It could be a gift. They just forgot to give you the key.
Really, I would call the rental company and tell them that there was one of their cars illegally parked and request that they resolve the issue with the car "renter since it has "flat tires" and cannot be driven. Chances are that if none of the neighbors are aware of the owner, and it has not been moved, there is something more complex. The rental car company should have a name if all is well.
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