Well, UK police is doing just that: putting balloons and messages in people's unsecured property such as homes, sheds, and cars. If they can get in, they leave a warning behind.
Now all you need is an opportunistic burglar (and aren't they all are), to follow in the police's footsteps and look for the balloon. Genius.
Still, brilliant initiative from the South Cambridgeshire police force. Modern, fun, and inventive. It's also getting some PR, which can't hurt. Good positioning, positive message, great brand campaign. Way to go.