I did a similar thing with a Gold Triumph Acclaim someone parked outside my Gran's house unlocked, which was more or less identical to my Gran's, which was usually in the garage.
BL keys seemed to be made in a limited number of combinations, my Mum once locked her keys in her Metro & asked to borrow someone else's keys & one of them worked!
The older Ford ones were similar, & the tibbe ones had a quirk where a key could lock any lock! The company car pool at the company where my Uncle worked fell foul of this a few times!