I've found them in my underwear drawer.
They're similar to my usual ones (plain black no VPL) but are a different size and a brand I've never bought.
DH does the laundry in this house so I have to assume he put them there when putting clean washing away. But, how did they get in the laundry in the first place?
We have two teen boys but no regular girlfriends afaik (older one has friends who are girls).
DH very occasionally goes away on business but they're not affair undies and even when I look for them I can't see any signs of anything untoward there.
We've occasionally given a key to tradesmen.
I can't think of anything else