I think I've read maybe 2 reports that someones FE was stolen.
And some magnets have gone astray. Many times the magnets showed up on other doors. Generally, I think it's kids just thinking their being cool.
I've decorated our doors with many magnets (and I mean MANY!) in our last four cruises. 2 of our magnets disappeared. If I wish, I can reprint the pictures that were on those magnets, so I'm still able to enjoy them. Yes, it's disappointing to think that someone else (especially on Disney) would feel that they could just help themselves to our stuff, but it's only a magnet.
We also use a wire wrap to fasten our FE to the holder. As to the stuff disappearing out of the FE, how would you know?