What should I do with my best friendfamily.

okay so for the past 9 months, I have been sharing a house with my best friend whom I've known several years and with whom I'm also really close with. She's a very bright person and she's stuck by side through lots of ****, she's basically my family. But I noticed something that worried me... So here's the thing, we do share our stuff but I've told her that I want her to let me know before she wears any of my clothes/shoes/jewelry etc. One evening she was in her room getting ready to go to her boyfriend's house for a sleepover and I went in because she needed my help finding an earring that had fallen behind her bed. The moment I stepped in her room I noticed her backpack was on the floor and one of my shirts was in there, I clearly saw. But I thought whatever, she probably forgot to mention it. She went to her bfs house (with my shirt in her backpack still I'm guessing). It's been like a week and my shirt was still not returned to me so I asked if she had seen it anywhere bc I didn't want to seem like I blamed her for stealing it or smt, and I expected her to just tell me she took it and forgot about it. But she didn't. She denied seeing it anywhere and she even said she'd look in her room just in case. Now the problem is clearly not the shirt I don't care if she keeps it. The problem is, why would she lie? Could it be possible that she put it in her bag to wear it later and she completely forgot? Or maybe she wore it at her boyfriend's house and she stained it with something and feels guilty? Idk but I just trust her so much and this was kinda like a slap on the face. It hurt me that she lied to me and it annoyed me that she took something without asking. What should I do? I feel like I've lost my chances at being straightforward about seeing it in her bag because I acted like I had no clue... Is there another way I could approach her, I already feel like my complete trust in her was affected by this... Why wouldn't she be honest?