The Chief's birthday cake with its invisible greeting.
Invisible icing for decorating cakes enables agents to celebrate special occasions in public without revealing their identities.
For example, the Chief's birthday at the 9474 Restaurant, where the invisible message reads, "Happy Birthday, Chief. From the gang at CONTROL".
The message is then read with specially tinted lenses disguised as earrings.
[Episode #33: "A Spy for a Spy".]