Seeing as the Secret Service is still in charge of stopping counterfeiting, they can just say they were there investigating reports of Milo passing fake $20 bills.
https://www.queerty.com/pro-troll-milo-yiannopoulos-denies-stealing-white-mens-charity-fund-20160822
Hmmm, I wonder if that would work also...
Question to walkingfeather:
Did O’s order specify Treasury side of secret service or protection side of secret service?
If it just said “secret service”, then it indeed could be someone investigating counterfeiting. Or an advance location checker.