I've done around 9 rackets in a row without getting a single stolen necklace. You figure theres 5 different items and 5 rackets you can do at one time, so mathematically, you should get 1 necklace for each 4 hour time frame. It won't be exact but should be somewhere around there.
I get a crapload of bloody mattresses each time but absolutely no necklaces. Does anyone know if theres a bug with the game or if it's setup, so the necklaces are very few and far between?