I found a key under the rear bumper yesterday. Looks like it has been there for many years. It looks original. Will go into the ignition switch but will not start the car. Will work on the doors. Any idea what might cause this? First picture shows what I found. Second picture shows the key on the right next to my current key. When I bought the car it came with one OEM key and one replacement key. I thought this was the second OEM key but the car will not start with it. It goes in the ignition and will turn the ignition key to on.
Thanks.

If you look at the DTCs logged, you will probably find P1260 because the key might not be configured to the car.

You can program up to 5 keys to the car.

^^ +1
My guess would be that whoever obtained the replacement key was also unaware of your find, so only the original & the replacement were programmed.

They must all be configured in one session.

When configuring keys, the diagnostic screen reads........... ARE ALL KEYS PRESENT? ANY KEYS NOT PRESENT WILL BE DELETED. DO YOU WISH TO CONTINUE?

Thanks everyone. I am assuming someone just forgot it was there.

It should read ARE ALL KEYS PRESENT (INLCUDING THE ONE HIDDEN UNDER THE REAR BUMPER BY A PREVIOUS OWNER)?

This is a valet key (you can tell by the color which is green for a valet key). As to programming, not sure. Valet keys are meant to allow starting the engine but not able to open glove box. There is a Valet switch (lower left of steering wheel) to enable/disable the valet function.

Maybe its my eyesight, but both of those keys look black to me. This is a valet key and normal:-

The extra key works all locks including glove box and trunk. My guess is someone forgot it was there and new owner felt they needed a second key so I have a non-OEM key and two OEM Keys. I've not been able to find a Valet key hidden anywhere.

Interesting, my valet key is dark green center and black rubber casing. (2002,XKR).

These keys have a crypto chip built-in the plastic handle. It is the size of a fat grain of rice. If you take a hard look at the top of a key, you should see the outline of a plastic plug. The chip is under it. The chip is there to prevent starting the engine with hot-wiring. The crypto chip has a write-only memory that can accept a numeric seed. From that seed, it can generate responses (basically calculations/bit flipping) based on an input from the key module. Keys have to be paired/programmed to the car by doing a "download" of that numeric seed with the help of diagnostic software running on the OBD port. All the keys (up to count 5) have to be programmed in one session to receive the same seed. The key you found looks original and could likely be re-paired to the others with the appropriate tooling.

Best of luck, keep us posted.

PS: You might want to blur the business end of these keys. Posting pictures of keys is never a good idea...