...He goes very still.
(Oddly, the Emperor is very sure this isn't a joke, or some sort of misdirection to distract him. The rumors about Caris were wild, and this is - moreso - but it feels like the answer to a question he hadn't quite realized he had, falling into place.)
"Who else knows?" He says this softly. Not angry. Very level. He hasn't moved a muscle but somehow his posture is not at all that of a pretty unimportant young harpist. (It's not the thing Altarrin does with his body language, it's subtler than that.)
Bastran is, meanwhile and without giving any external indication of it, reaching out with the communication-spell; there's a gap in this shields for it keyed to his own magic or to a particular rare and secret version of the spell, and one of his guards is probably stationed right outside the magically soundproofed door. He knows it makes them...nervous...whenever he takes a new lover in private.)
<Maximum alert> he sends; they'll set all the wards and alarms to the most sensitive parameters. <Don't intervene yet but - check in ten minutes, if I haven't interrupted by then>
It's sort of the bare minimum precaution to be taking that isn't just being completely negligent, now that he knows the beautiful man in his room might have unknown capabilities that he can't detect, but - his gut, which can of course be tricked by sufficiently good acting but is pretty acute, still tells him that Caris isn't here as a threat.