She could have. It's not out of the question. Maybe her time of the month came early or something and she needed a new pair. But it seems strange. And it also seems strange that any of the roommates would have underwear lying around in the "common area." I don't know if there is a laundry room where he could have grabbed them from. Or, more frighteningly, maybe this perp knows KT or one of her other roommates....maybe he was intimate with that person, and that's he has the underwear.
But why? Why did he take a pair of underwear obviously from that house and use the black thong? This is sounding more and more planned/taunting, whatever you want to call it. And it's also sounding more unbelievable that no one else in that house heard anything strange going on that early morning....