But cash can move around quite a bit in a day. Spend $14 at merchant A, get $6 in change, spend $3 at merchant B, who hands that out to customer C as part of their change, who gives it to a bum, who in turn uses it to buy lunch... seeing a deposit of a suspect serial number at a merchant doesn't mean a whole lot. At best it gives you one more place to look through their CCTV and not find the guy you're looking for.