Yuuri and Victor are fucking, but someone is watching them (maybe they didn't shut the door all the way, or they didn't close the curtains, or they think they're alone in a public place but they're not). Yuuri - and only Yuuri - notices that he and Victor are being watched, and Yuuri takes the possessiveness/exhibitionism to another level.

No preference on who's watching them! I like that it could be anybody.