Not so far fetched

This happened at Alton Towers, a place where you go with your family and often end up, up-side-down. Consider many families have multiple cars.

Thus, it's not inconceivable for someone to lose their keys at Alton Towers, and drive the person's car to their home (providing you can find the car in the car park, or maybe the keys were 'lifted' and you were followed from the car park). You would then spot a Saab, Merc, BMW etc. key on the ring and consider going to the person's home worthwhile.

When you arrive there you may find multiple cars matching the keys you now have (does anyone else carry their wife's car key on the same ring?).

For me the satnav isn't a big deal, especially if you have anything else in the car with your address on.