When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
If that round black thing was a button to operate something, then the mystery fob would have to have a battery in it. I have seen small keychain led lights about that size. Some you can open and change the battery, some are pure disposable.
Well, it's in the glove-compartment now. If I should ever need it, ....
Good thinking, now if someone comes up to your car and offers 100 Euro if you can produce a worn-out old and slightly disgusting old keychain, you'll be glad you didn't just toss it in the dumpster.