Sebek is described as being “inattentive” by Jamil when Sebek accidentally follows a group of students to Scarabia after the opening ceremonies, as he somehow did not notice that Scarabia was not Diasomnia.
Sebek says that Lilia has also told him that he should be more alert but “I thought I glimpsed Malleus outside, and I was so happy that, well…”
During Halloween Silver realizes that Lilia was not possessed and Rook realizes that Malleus was not, but Sebek did not notice at all.
There are many things that Sebek does not notice during Halloween.
Trey says that Sebek gets “serious tunnel vision” when he’s worked up, saying that he first noticed it when they “were fighting the intruder at Ramshackle”. Sebek also does not seem to realize how uncomfortable he makes Rook with his line of questioning about Rook’s family.
Trey distracts Sebek from the invasive questions by pretending to see a ghost behind a pillar to protect Rook’s privacy. (Later on, Sebek also fails to notice a door that Rook ends up having to point out to him.)
Despite being associated with Lilia since he was six years old Sebek does not recognize Lilia when he delivers the Spectral Soiree invitation to Ramshackle (or that the invitation itself was written by Malleus).
Twisted Wonderland Reference Note 