It’s happened to me twice.
The first time was at a party when an ex-class mate told me she knew I pooed my pants at school but was to scared to tell me at the time. She was very drunk and made it extremely obvious she liked what I did and wanted to know if I still did. This was in front of everyone at the party so I of course I said no, yet she insisted on checking the underwear I was wearing to see if I was telling the truth. She went into great detail about how she knew, what she used to do to check and just how sneaky she had been.
The second was a friend’s daughter who asked if she could keep the underwear she had taken from my washing basket. Despite being told to be quite by her parents she explained how she wanted me to stop doing poos in my pants because she liked how my underwear smelt when I had been wearing them for a while, like the ones I had been wearing that day. According to her they were exactly what she wanted but not if I had pooed in them.