Since this has been a major topic since last chapter, I'll try to give my reasoning as to why I didn't feel that it wasn't completely out of nowhere (or just my justification of it).
Please note that I am not the author, and this is just how I perceive it, so you should take this with a grain of salt.
Case: The Defendant, Tweight the Shadow Hero (lol), who is supposed to be undetectable, was detected twice in the span of the last chapter, while failing to detect that he was being detected, except for when he did detect that he was being watched in this chapter (he wasn't trying to be undetected this time).
Exhibit A: Detected by Elysia
While the defendant was allegedly stalking the Student Council President (it was for a good cause), he was detected by Elysia, who was lying in wait.
Let me begin by recounting the turn of events in detail. The setting is within the academy, and the defendant was waiting for the Student Council to leave so he could stal-, I mean, observe them. When the Student Council left the room, the defendant followed at a distance away, presumably undetected by the Student Council. However, before he could leave the academy, he was caught by Elysia, who predicted how he would act, and waited for him.
Defense: There are two scenarios I can think of to defend why the defendant didn't detect Elysia:
1. The scenario where the defendant truly did not detect Elysia.
Obviously, he didn't notice Elysia because she's the losing heroine. /s
Yeah, I don't know how to defend this scenario, if it is the case.
2. The scenario where the defendant did detect Elysia, but did not think it was Elysia
For a more serious answer, I believe that the defendant did detect someone's presence, but didn't necessarily conclude that that someone was Elysia, after all, the defendant assumed that Elysia had already gone home.
Since they were still at the academy, there could have been other people around (i.e. students, teachers, staff), and since the presence wasn't hostile (or made use of magic, like in the case where the defendant detected the President), the defendant could have assumed that the presence was just someone affiliated with the academy. Since the defendant is a student at the academy, it wouldn't be strange to see the defendant running around the academy.
Therefore, the defendant could have concluded, "Since that presence isn't hostile, they are likely affiliated with the academy, and so, if they see me running around the academy, I can make up an excuse. After all, I am a student here; it isn't suspiscious for me to run around the academy." However, since Elysia had an inkling of what the defendant was up to, no excuse would have cut it.
I would like to add that if the defendant had known that the presence was Elysia, that would truly make him a creep, and I would take back my defense.
Exhibit B: Detected by the Knights
I agree with Itdome and WhimsiCat's explanations above, though I would also like to add that, even if he was seen, he wouldn't be thought of as someome suspicious, since it wouldn't be that strange to see a student wandering around the town.
TL;DR: Tweight could have been thinking that, even though he could be spotted by bystanders, he was dressed up as a student, and it isn't suspicious to see a student walking around the academy or the town (after all, even the student council was walking through town). Since none of the gazes were particularly hostile or observing him with magic, he didn't think of those as a threat.