It is a good idea to attach your contact details to your car keys, should you ever lose them and a stranger picks them up.  It can be in the form of a keychain, a cardholder, or even on the key itself.  On this keychain, you should have your name and contact number, and even an extra number if you cannot be contacted.  Now you will have some peace of mind.  Do not put anything else on this keychain, such as a work address or a home address, as this puts your personal safety in danger, and could make criminals target your home.