After 9-11, I was tripped up with the SSSS on my boarding passes several times but I haven't seen one in a few years. In the past, it was just extra degree of reviewing my documents and opening up my luggage for them to review.

But today, they don't even check your boarding passes after you walk through the metal detectors so not sure when TSA would check your boarding pass again.

Sounds like the original OP had a secondary check after boarding by TSA as this was international flight to UK. Not sure why they would swab her or do the patdown for a 13 year old, just sounds weird. Perhaps her name got caught in database but doesn't explain why the inward bound was a non-issue.