Keyless entry by RFID

My new car has a keyless system whereby the car senses the proximity of the key in my pocket and lets me open the door or start the engine at the touch of a button - but only if the key is within a few feet, so thieves and vagabonds are kept at bay. The 'key' that enables this is reported to cost between 100 and 200 GBP to replace if I lose it. That's scandalous.

For many years shops have been using RFID tags on merchandise to raise an alarm if an item passes too close to the sensor mounted by the exit door. The system cancels the alarm for a paid for item, and the tags are cheap enough to be used on even very low value items, which means RFID technology is sufficiently baked.

How about a keyless entry system where the smartness is in the lock/door/etc. and can be set to recognise a collection of (really very cheap) RFID tags for activation.

Instead of requiring a ridiculously expensive 'key' I could list scores of RFID tags. The tag doesn't need a battery and takes up negligible space. If it wears out then it's easily and cheaply replaced. It's sufficiently small and cheap that I can have one stuck on each belt buckle I own, another on each watch, another on my wallet - all things I don't normally leave home without.

I'm not suggesting automatically opening doors or anything - that adds another layer of complexity in the control systems. This is just a straight replacement for a standard lock, only instead of having to carry a key I could just press a button to lock/unlock.