How should I address her?

So I’m pretty sure my suitemate has been coming into my room while I’m gone and taking some of the masks I have hanging up on my walls. They are for when I go to class in the morning. Mind you I only have one class but I put like 30-40 masks to hang up now there are only 2. I have began to lock my door. But I forgot last night and saw her out with a black mask on. It didn’t register to me that it could be mind til later because she was right be hind me and pretended not to see me nor did she speak. I don't know how to approach her because she always has a ton of people over so I didn’t want to address it in front of them. Should I just text it to her? Or ask in person whenever I can? I don't know I’m pretty soft spoken but this is a violation
How should I address her?
Post Opinion