For mine, it's bloody USB charging cables!!! She comes and nicks mine out my office, then plugs them in to charge her iPad/Phone/Speaker/whatever (We have electrical outlets on the walls with built in USB sockets), then leaves them lying about in the living room, or where ever she used them. In the mean time, I can never find one when I need one, so I go to our local pound/dollar store buy a bunch more, put them in my office, then when she wants one she comes and raids my office again. I looked downstairs in the living room and within 30 seconds I found 5 under the sofa, another 10 under chairs and behind furniture and 7 in the drawer where she keeps all her misc junk, that's 20 cables all within less than a meter of where she was sitting/sits and she still comes trudging upstairs to my office, and just takes cables off my desk. Drives me bloody nuts. I've actually use a hot glue gun to seal around one of my charge cables where it's plugged into the back of the PC (So it's effectively glued to the PC) just so I have at least one she can't take...