Reaper had gone insane from grief and exhaustion; That's the only explanation.

Why did he believe that? Many reasons: things (cups, books, random household items, etc.) were suddenly misplaced regularly, food was going missing from the fridge, occasionally broken porcelain decorations littered the halls, the scent of a chocolaty coffee often filled the air late in the day, and eerily enough, sometimes he would come home to find Geno's half of the bed startlingly warm.

All of which seemed to be signs that (A. Reaper missed Geno so much he was imagining things and moving/breaking stuff around in his sleep-deprived haze or (B. there was a lovely bloody ghost haunting Death Manor (as unlikely as it may be).

And nothing listed above could be blamed on Goth either. The young death god had long since moved out and took up residence in Aftertale, causing quite a lot of drama since no one in the AU remembered Geno. (Thus, leading many monsters to believe he was the resident Sans' secret love child- which wasn't completely inaccurate.)

Nevertheless, the raven-winged God of Death was determined to locate the cause of these strange phenomena. Whether it be by taking an hour from his already busy schedule to visit a psychiatrist or setting up cameras in his house in order to catch evidence of a nonexistent ghost.

Naturally, the latter was his first pick.

Imagine how confused Reaper feels when he catches video of the Destroyer of AUs waltzing around his house like he owns the place.