Originally Posted by ronaldo

Give her a key and change the locks. Or, give her a key she THINKS is your house key. Unless she's planning on moving in with you, when would she really need to USE it, unless like has been suggested, she wants to snoop.

One day she'll tell you the key didn't work. Play stupid and say you have NO idea why it doesn't work, and ask her what she wanted in to your house for anyways.

Hopefully that'll give you a clear understanding of her motives.

Or, just give her back her key.