Glasses of tea and water left out were definitely possible places for her specter to pop in and out of. He might consider that if he doesn't want accidental spying. Yin will, especially, spy on people she has taken a liking to. Waver being one of those individuals.

Yin's expression tightens, barely, so slowly, and by a mere fraction, but the concern is there.

"What happens to the person on the other side? How do they get back?" It sounds treacherous. With someone replacing the original person, no one would know, unless, of course, they realize the person was acting different. With Yin, would they even know how she's supposed to act?

She nods to the advice. She'll find the mirror with her observer spirit and put a cloth over it right away.