So I'm playing black 2, and I'm in the black tower getting money and leveling up my evd gengar. So I finish, and I notice that his focus sash is missing... It's not in my bag, it's not in being held by a Pokemon in my party or in the box.. So, wtf happened :-(
The only possibilities I know of:
Maybe it was consumed when he took a usual one-hit-KO attack as it is a one use item
Have you tried looking in your PC with your name on it.