My sister did that once and was about to drive off right about when we both noticed the car seemed a lot cleaner inside and “since when do you wash out the ashtray???”, about simultaneous with the owners showing up too.
It was to damn funny, two champagne edition VW rabbits and the keys fit and started it. Turns out around 77/78 the average VW key would fit something like every 12th one made...
When I was a kid, my parents had two Chevrolets that the same key would work with.
I used to drive a gray Mercury Sable. That car was so non-descript that I often would be trying to get into Taurus, Buick’s and other gray cars that were parked near mine. Succeeded a couple times.
I got a great deal on that car. Bought it with 90K miles for $1500 from a friend who was moving overseas. I drove it for 3-4 years, then my daughter drove it for 2, then my son had it for 2. He got rear ended last year by a guy who offered me $4000 to not go to insurance.
No problem!