Just to further clarify the issues with being anonymous

Passenger At check-in -- "we were upgraded? Why"
Agent -- "Upgrade certs were applied"
Passenger -- "Huhh?"
Agent -- getting suspicious, "you don't know who applied the cert?" (or the agent thinking they got applied to the wrong passengers)

Probably will work out fine, but agents are on the look out for fraudulent use of certs and there may be a few awkward moments depending on the knowledge level of the passengers