Have you recently sold or returned an iDevice?
EDIT: Might be this:
http://gizmodo.com/5880593/the-apple-bug-that-let-us-spy-on-a-total-strangers-iphone

Just got off the phone to apple and they have confirmed it and reset my password. He still could not give an explanation though. Hmmmm tempted to leave iMessage off now, seems its rather vulnerable.
This sort of thing has happened before as well...