I was gonna start a thread on this then forgot.

There was a pair of boxers on my clothes horse that looked unfamiliar but I initially assumed were new and I forgot I had but when I tried them on they definitely didn't fit me.

Given I live on my own, have had no-one stay in my flat for yonks (and if they had I presume they'd have taken their underwear with them) and I have my own washing machine and no-one else can acess it, and these were newly washed, I can't figure out where I acquired them from...

What's the strangest thing you've found in your flat that isn't yours and you can't account for?