After checked baggage is screened, TSA requires the TSOs doing the screening to place a mark on the baggage tag itself to show that it's been screened. What, exactly, this mark is, is up to the specific airport TSA offices to decide. I've seen it where it's just a slash, sometimes it's an 'X', sometimes it's actually a stamp or a sticker.

In Huntsville, we use a red (we've experimented with yellow, black, and blue, as well, but red shows up the best) Marks-A-Lot marker to write a number on the baggage tag. The TSOs are all assigned numbers (kind of like the old "Inspected by 12" stickers you'd find on some manufactured goods for QA purposes) in Huntsville, and the only thing this number is good for is to denote which particular TSO screened the bag. The supervisors make rounds through checked baggage, checking the bag tags to ensure both that the TSOs mark them, as well as mark them legibly.

Incidentally, this number is also written on the lovenote that the TSO places inside the checked baggage if we have to open it, typically alongside the time and date stamp that we put on them.

So if you were flying out of Huntsville, say, and you noticed something was missing in your bag, and you called the TSA office to make a complaint about it, they'd first tell you to look at the bag tag and tell them what number it is. Then they'd ask for the timestamp on the lovenote, and then we have a wonderful 100% full-coverage camera system that records everything that happens in baggage.

All of this was done as a deterrent by our TSA management to clear us of wrongdoing. Supposedly, it's already exonerated six TSOs who the passenger was just absolutely certain had stolen something out of their bags.

Our system works splendidly without causing any kind of noticeable delays in screening, but like I said, it's up to the individual TSA offices at the individual airports to decide what kind of system they use.

However, yes: TSA does require us to mark the bag tag itself, in some way, to show that it's been screened.