the moment the series decides to remind you that in spite of being stupid OP and having answers for everything...he's still only a young human, and as such no matter how he outmatches other humans or lesser demons he's still got limits...for now...
@gmayor61 - the plot developed the same way in the novel. It's time for a climatic battle, but not the end of the story. @StaticTransit - most likely, the demon noticed he was found, and forcibly broke the seal a little earlier. @Wolvenworks - well, he can beat that demon lord. He just needs the right sword.
@StaticTransit - well, pretending to be sealed is pretty safe. The seal may act as a way to hide your trail, to deflect suspicion and so on.
Also, forcibly breaking the seal is probably hard and might result in lasting damage, while waiting for it to weaken doesn't.
@flannan If he's as powerful as he's made out to be, I don't think he really needs to worry about playing it that safe. If he's as super powerful as they say, he should have broken the seal earlier and gotten the jump on them.
@StaticTransit
We don't know how much he knew from within the seal, though. For all he knew, there might be super OP people still around, just hiding like he was. Someone sealed him in there, after all. That might still be possible.
This feels more like the demon counter-detecting that human magic has declined enough. Anyone capable of detecting him would probably be the pinnacle of humanity, and if he traced that back he'd know Matty isn't all that powerful without equipment. Relative to the demon, of course.