Just like Boy with Apple, the ever-so-secret Society of the Crossed Keys makes a few cameos before it's ever officially mentioned. We see the Crossed Keys emblem below M. Jean on the concierge desk, on the shoulders of Gustave's purple vest, and curiously emblazoned on Agatha's necklace.

The emblem is, of course, representative of the secret society itself. Keys because a concierge holds the room keys for the hotel (and apparently at some point people used actual keys—who knew?). More abstractly the keys may represent a type of power and responsibility held at the concierge position, the ability to lock and unlock each room and each guest… and the responsibility not to.

The crossing of the keys must be symbolic of the brotherhood shared by the Society's members. Their loyalty not only to their respective hotels but also to one another is obvious due to their willingness to immediately pause what they're doing and help out the stranded Gustave.