Favors should match the theme of the party, so consider giving detectives a mini flashlight, latex gloves, handcuff keys, and a notepad and pencil for crime scene notes when they arrive.

Each detective will use the latex gloves (you can buy a pack of a couple hundred from the hardware or drug store), dark glasses (sold in bulk at the party store) and a paper bag to collect evidence (in this case candy gold spread throughout the "crime scene").