bpfsr:

As a former banker, the only way that I know of determining where a check was cashed is to manually examine the endorsements. To the best of my knowledge, in the present system there's just no way to computerize it.

"The Letter" is more likely tied to the frequency and amount of orders (ie, too often and for the same amount). Computers are good at spotting trends like that and I'm sure Chase's are no exception.