It is cute that you think there's any chance in hell that such a service would be granted permission to fly drone-spotting drones by the FAA.
In any case, a single 1.8 gigapixel ARGUS-IS can cover a huge amount of surveillance area and within a few years that technology could be commonplace, so the answer could easily always be "yes, of course there is a drone watching your car right now", making that information basically useless.
No, the FAA wouldn't prohibit amateur RC planes, but how exactly do you implement a drone-spotting operation using amateur RC planes? Even ignoring the complexity of coordinating such a thing, the short range such planes have and the need for constantly re-powering the planes, what sensor do you mount on amateur RC sized planes that is capable of spotting a drone flying at up to 30,000 feet?
In any case, a single 1.8 gigapixel ARGUS-IS can cover a huge amount of surveillance area and within a few years that technology could be commonplace, so the answer could easily always be "yes, of course there is a drone watching your car right now", making that information basically useless.