In class we were talking about 掏り (pickpocket(ing) when the teacher drew 掏摸 on the board. Why would something that is based on a verb (掏る) be written like that as a noun? I resisted the urge to interrupt and ask, because I do that about 100 times a day already with my silly questions, but I resolved to find out the answer myself.
WWWJDIC claims it is gikun, but my understanding is gikun is when the reading doesn't match what you expect from the on or kun'yomi, like in the case of 今朝. So is it ateji? Wikipedia seems to agree with that theory, but I discoved WWWJIC is also claiming something from that article is gikun! Maybe some talking with Jim Breen is in order, or I may not understand gikun yet...
Anyway, why are we sometimes seeing those kanji? Well 掏 meant pickpocket to begin with, and 摸 kind of meant to grope around, which is what you do when you lift a wallet. To complicate the whole thing, the sound came from 擦る, which is to rub, also for the bump and pump that a pickpocket does. All of this entymology is courtesy of the fine folks at Gogen.
It's no wonder that most people just resort to katakana (スリ) for this one.