What do you do, when you know someone is using your apple ID?

Has anyone ever gotten one of those creepy notices from Apple, that says that someone has signed into FaceTime using your apple ID...and it wasn't you?

What do you do, when you know someone is using your apple ID? I feel like it could be a police matter. What if some creep who likes kids is using my apple ID to do whatever sickos like him do, with my ID? I'm going to save those emails!