How do people get their knickers under a rug?

This is not a rhetorical question. I mean, literally, how would one lose their knickers under a rug? And this rug is not a little thing that tends to shift. It's a huge (3m x 4m), heavy rug, that sits on top of wall to wall carpeting.

I was vacuuming today after my T left and decided to lift the edges of the rug and vacuum there too. Among various other things there was this item...Clean and in good condtion.