It used to happen only at smaller airports that didn't have the capacity to handle extra lines. But lately, I've had it happen at bit airports (like ATL), so I'm not really sure.
The answer to your (implicit?) question is that it doesn't work. The staff take the card from you at the start of the process, and once your bag is in the machine, they have no idea if it's yours. So far, every time this has happened, I"ve been singled out for additional security.
So after you already start the x-ray process, they take you aside and make you do the shoes, belt, and the rest?
If I sound confused, it's because I am :). I don't get a pre-check card, it just shows up as a notation on my boarding pass. I scan it just like normal, then shove my phone into my backpack and toss it on the belt. No removing my shoes, belt, etc.
They also won't touch my bag, and I have to be the one to push it onto the feed belt of the scanner. They have to have a pretty good idea that it belongs to me.
Coincidentally, I just travelled through ATL a few weeks ago. It's never as smooth as my home airport (PDX) because it's a much busier airport, but I don't recall anything out of the ordinary happening.
> I don't get a pre-check card, it just shows up as a notation on my boarding pass.
(Not the person you replied to.)
These are different things. It has nothing to do with you or your eligibility, but the specific time and place.
If you're eligible for PreCheck and have it printed on your boarding pass, at some airports and security lines that don't have a PreCheck lane open, you're given a laminated card that the first agent tells you will accord some of the PreCheck benefits. As with the OP, these didn't always work in practice in my experience.
The PreCheck line can temporarily be closed in a larger airport that would normally have one, or this may be a more permanent process in a smaller airport that might not have the space.
My local airport is so small that they don't process security until half an hour before boarding. We have one "gate", and the planes we get can hold 50 people when full.
I have Precheck (always marked on the boarding pass and I assume it shows up on their screen when I scan it), and what happens is the person at the podium notes it and lets his colleagues down the line know. I get a card, and show it to the person handling the body scanner, at which point they send me through the metal detector instead. There isn't much difference when it comes to the bag scanning (if any).
The answer to your (implicit?) question is that it doesn't work. The staff take the card from you at the start of the process, and once your bag is in the machine, they have no idea if it's yours. So far, every time this has happened, I"ve been singled out for additional security.