Upvote:8
It is unclear what you mean with ‘red flagged’.
You seem to assume this is a standard term, but it is not - unless you mean an umpire literally throwing a red flag at your feet.
If you mean getting an ‘SSSSS’ written on your boarding pass - it is mostly random selections. I got two of them in about 300 flights over the last ten years - it doesn’t mean anything.