Though its certainly technically possible, I haven't seen this happen on UA since prior to the merger. Even then, its so infrequent, I think I only needed to do it maybe once. At that point, typically, there would be a note in red on the PMUA emails noting the CC was required to travel.

Even if there was a flag on it, simply showing up at a check in counter anywhere in the system (or a UA ticket office..in the places it still exist) and showing them the physical card would remove the flag - could be done anytime prior to departure.

IMO...this is something they do more commonly on European and Asian carriers - have had this happen a few times (TK, TG, CX, SQ, maybe others) when using my US card. Haven't had it happen in the US in many, many year.