Maybe it's not amazing that the letter writer thinks that the solution is yet another letter?

Agent A: “hey Bob, this one has something medical-looking anally attached. Should we get a medic to take it out for inspection?”

Agent B: “No, they’ve presented us with a Doctor’s Note. Let them in through.”

I suggest that any patient with a seton who is planning to travel to the USA or any other country where they are likely to be searched in this manner should carry a letter from their specialist explaining the nature of their condition and treatment.