Its so much harder because you are a girl lol.
A guy would be getting his assed kick all over the apartment. In fact he would probably never come back at all if we called him and told him what we found.
Personally I would tell the landlord, call the police and have one come over and document it (without pressing charges) You can file without actually pressing charges you know lol.
And it gives you more credibility.
After you have done that I would call her up and tell her what you have found before she comes home.
The situation going back over the semester makes it seem as if you all have confrontation issue. Meaning you don't like to do it.
If that's true, well you suck lol, but you can always get someone else to do your dirty work for you and call and deal with her even in person.
But honestly, not only is that the height of bad manners, its shows a great deal of personal cowardice lol.
However, it is more important that you get her out of the house at the moment than you overcome a life issue.
After all, you don't want her to get some strange ideas and start acting out her panty sniffing fantasies on you lol
.
Although, unfortunately, there is probably a more reasonable answer as to why she possess your filthy undies lol.
Have a Happy New Year!