"It is impossible to make anything foolproof, because fools are so d*mn ingenious."...
Likely the best insight we can have as to the product design team's vision is to look at the marketing. Looking at the page on Apple's site, it seems they're marketed as a tool to help find stuff you've misplaced or left behind. They also apparently did have some thought to misuse given the initial features alerting to an unknown airtag nearby, a capability they're enhancing further.
I don't own an airtag - how long away from your phone before it begins beeping or alerting others? I'd understood it was several hours. How much shorter of a time is it?
I'm also curious about the keys left for car repair thing... Maybe I'm too old school, but I always would leave my spare or valet key with the mechanic. I still needed the other keys on my keyring for door locks and file cabinet locks. Do people really leave those other keys with their car when it's being serviced?