This is horrible, but when I was doing research on other crimes yesterday I found other cases in which the perp kept underwear as a trophy.
I think that either:
Pasty
Burke
or an intruder
put them on her. So, yea, I can see the trophy aspect to it.
highly probable - a lot of pedophiles start out that way - stealing little girl panties from apartment complex wash rooms, etc... Even man rapist stalkers do the same thing to their victims before they strike.