You are treating your household members like a family. That is a plus factor for you. Here comes this feeling of "care" towards her and you want to do something about it because it is not appropriate. Women have the uncanny ability to sense things and they are usually right!
You have to talk to her. If there is a language barrier, then use body language. Another option is, establishing a closer relationship with her. Sometimes it is good to act (actress-like) to catch a culprit. Sounds funny but I did it to people when I am in doubt. But still, confronting her is the fastest way to solve the mystery. If she denies the whole thing then so be it. She is entitled to privacy rights and if something happened to her along the way, blame not yourself because you've done your part.