Well, then, I suggest something more "obvious" than leaving a fob (that doesn't need to be plugged in) in a cupholder. How about putting it in a plastic baggie and tie-wrapping it to the steering wheel before you get out? Ie, you want it to be so obvious the key is there than no one can argue that they didn't see it.

Meanwhile, I think the fleet suppliers to rental companies need to rethink which car-starting technologies make sense for rental companies and which make sense only for consumers. IMHO, a fob like this would only make sense if it could detect whether it was in the possesion of the renter or the rental company, and if could detect whether it was in the rental facility or not, and issue different alarms based on those combinations. A fob designed for a single user for the life of the car ownership (or lease) doesn't work well in a rental environment IMHO.