How to secure water fill inlets?

I really can't think of a reason why anybody would need to secure there water inlet but I have to admit I had a trailer with a locked water compartment and my new Rig did not come with one and I thought that was odd but really did not think why... I can imagine if your neighbor was po'd at you and was trying to get even for your Dog pooping on his side of the line might want to get even but if that happens you probably got more to worry about.

That said I lock my doors when I am gone and at night and have a video security system that emails me when it sees movement. Some of my friends think I am Paranoid but I think it's experience from my years in LE now retired...