Good theory, but no, no washing machine.
And I checked if I put both socks on the same foot too, but that wasn't it either.
Actually the two socks on the same foot got me thinking maybe the pantie thief wasn't actually a thief but just made an innocent mistake.
Maybe he accidentally put on the girls undies after the session in addition to his own; maybe in the dim light and his post coital bliss or the rush to get out on time he wasn't thinking straight?