Son (15m) taking bedwetting pull-ups from younger sister (11f). Unsure, why or what to do.

A couple months ago, my daughter told me (40f) that she felt like her drynites (bedwetting pull-ups) had been going missing from the pack. (For context, she wears them every night due to bedwetting issues). We discussed it but i didn't think too much of it, but she kept bringing it up. Apparently, after she started to notice, she started counting them. And she's completely convinced that they are going missing. She says they go missing every now and then rather than ever day or something (like one or two times a week) but it's sometimes multiple each time.
I really wasn't sure what to think. In our house, it's really just my husband and i, and our two children. I really don't know what anybody other than my daughter would do with bedwetting pullups.
My son used to have bedwetting issues and wore similar protection until he was about 12-13 when it seemed to have gone away.

I haven't specifically looked for them, but i will admit that i've seen a couple in odd places. My daughter has a bin in a bedroom that she uses specifically for them, yet i often see used ones in various other bins around the house. I also have found some in other places, including the laundry basket (once, yes it was wet.) That was before my daughter told me they were going missing. I initially assumed it was her and basically just told her not to. She denied it at the time but i assumed she was just embarrassed. Thinking about it in this context, i do wonder a bit. I also found a very destroyed one in the back garden stuffed between the fence and our shed.