Old roommate help!!?

so my old roommate moved out two weeks ago due to getting caught stealing my things. Through the course of us living in our place she had several keys and "lost" them. Since she's moved out she turned in one key. The other day my new roommate came home and the front door was wide open she said. I went to go check the mail the other day and the door was unlocked and just thought my new roommate used the door and didn't know how to lock it. But last night she told me she came home and the front door was wide open, so I got up and asked her if she's used it and she said no she hadn't touched the door, I got up and started playing with the door and began to tell her I never forget to lock the door and there is no way it can unlock itself and open. So I came downstairs to get a glass of water and I sat down on the couch and I felt a cold breeze and I look over and my door is unlocked and wide open. May I remind you I locked the door when I was messing with it just last night after our conversations. What can I do about this? It's bull that she's being an immature kid and going to the level to play with my house... What can I do about this?