I'm in two minds about this situation.

As you guys know, my cats are inside/outside and so I don't think a cat should be stolen just because he/she has some freedom to wander.

But on the other hand, even I keep my kittens in doors. Kittens need to be indoors until they have grown a bit and been altered.

But whatever you decided, please be direct with the owners, it is the only fair thing to do.

If they care enough to tag their kitten, how bad would they feel if she went missing.