Didn't sponsor upgrade for anyone flying same day. I could see how wires might get crossed there, but the alerts did show the confirmation for my reservation with the bag tracking number, so I think it was truly (mistakenly) checked to my record.

I guess maybe someone with same last name but different first name could have been on my flight, leading to initially wrong association, but one that was noticeable afterward from looking at baggage stub. If it were fully the same name, I can't imagine the other "me" would have noticed at all.

I'm surprised someone looked at the stub closely enough to notice, if that's what happened. And I guess that proves how closely some agents check ID when accepting the bag!