[ Seimei taps on the glass, almost like he expects to be able to taunt the potential hallucinations into revealing themselves again. It doesn't work, but he still looks satisfied with himself after testing the mirror. ]

Oh, we can't know unless we try, right? Even if they were put in there, how would we know it was for a good reason if we don't test it? Maybe they want out and can't tell us. That's why we have to be the ones to look first.

[ It could also just be an illusion made up to trick them, with no actual entities involved at all. There's always that chance, but Seimei will still happily try to bait the spellcaster behind it just to lure them out. He's never been bothered by using himself as a trap. ]

Did you see anything else strange so far? Maybe it's a clue.