One of the guests is an emissary from one of the Faerie Courts in magical disguise. Not because of anything sinister, but because among the Fae it is considered exceedingly impolite to show one's true face to any but your most intimate friends and family. Everyone of importance is aware of the emissary's presence...but if the PCs detect a magically disguised guest, they might start an international incident. Bonus points if they catch wind of the assassin, and then accuse the Fae of being involved in the plot. Hope one of your PCs has Know: Nobility. Or Nature. Further bonus points if the assassin is actually magically disguised as well.