Still, inquiring minds do inquire.. but lets not tell the world..Any RF signal can be captured and reproduced. You don't even need a physical fob, just the chip, which can be added to the vehicle via OBDii.
My understanding is......... only so many keys can be added to any vehicle, let's say 7 and if all seven slots are full, the system can be erased wiping out all 7 slots and a master key is the only way to add keys back into the system (a master key is one of the keys originally programed when the vehicle was made). If a non-master key is attempted it "bricks" the system and the security module must be replaced. That's why you must have all keys fobs present when adding a credit card key in order for existing keys to continue to function after the CC key is added.
That being said, my theory is: if funds were not an issue one could purchase the max amount of keys and have them programmed to the vehicle, thus filling up any available slots and severely hamper a surreptitious key addition. True professional thieves have another way to steal your vehicle (I won't discuss this method on this forum) and there's not a lot you can do about it.