You shouldn't steal a woman's underwear.
Bill sleeps with a woman at her apartment, and she tells him she is leaving the country forever the next day.
Bill wants to have something of the woman's to remember her by.
Bill asks the woman for something of hers to remember her by, and she gives him a lock of her hair.
Bill takes a pair of the woman's underwear without her permission and takes them with him when he leaves.
The woman thinks Bill is romantic for wanting something to help him remember her.
The woman finds out Bill took her underwear and thinks he's weird.