Not sure if it was covered, but what happens if the car is started and driven several miles away while the 1 key is left at the house? Can the vehicle be shut off and then after 5-10 minutes or so, can it be restarted and driven back home? Can you lock it and leave it, then access it with the keypad and start and drive it away without the 1 key you have being anywhere near the vehicle?
Did you get any carfax history on this vehicle? Any accident/flood/salvage/fleet usage history? Any history on prior owner? Was it a demo vehicle?
People do strange stuff with their vehicles. It wouldn't surprise me if a fob was hidden in the vehicle as jmr said earlier so the owner never had to carry it with him and presumably just used the keypad to gain access. If your dealer/locksmith has a "sniffer", I would assume they could quickly check the interior to see if they can locate the fob without having to tear anything apart. It also wouldn't surprise me to find out the car had problems and that is why the previous owner got rid of it.
Best of luck.