From my experience TSA does want to know anything they see on the scanner that is out of the ordinary. I used an external catheter a few years ago and the urine in the bag was recognized. Despite me having a doctor's note explaining that I needed it, I had to show it in an examination room. After I emptied the bag I was allowed to continue. A diaper doesn't always show but this year it did. I tried to change before going through the checkpoint but the airport is always quite stressful to me, shuffling from one queue to another. So this year it was so wet it showed as an exclamation mark at my crotch on the monitor. Again I had to go to the examination room. I had to lower my pants and a swab from the outside of my diaper, front, downside and rear was taken which is used to check for drugs. Probably they have seen people before smuggling drugs inside their diaper, but for me it was very embarrassing.
Obviously everything was alright and once they determined my diaper was just a necessity they didn’t care about me anymore.