Are you 100% sure? My car (12yo Toyota) has keyless entry and start, and the fob has an RFID chip inside. If the car can't detect the fob (e.g. the battery is dead) you can hold it against the "Start" button which has a RFID reader. I ask, because this feature is not advertised other than a few sentences in the manual, and it seems a pretty badly designed feature to not have a backup.
The fob also has a physical key hidden inside - although that's a bit more obvious as there is a key hole on the drivers door.
The fob also has a physical key hidden inside - although that's a bit more obvious as there is a key hole on the drivers door.