I can't imagine being the test subject in this study:
"Hey, how'd you get 100 bucks?"
"Well, my doctor was operating on my abdomen and they hid sponges inside of me before sewing me up. Then they tried to find them. Sortof like a scavenger hunt."
Better use a metal detector, too...imagine the doctor going out to his car after a 12 hour shift only to find he had "misplaced" his keys.