It's like that usually in my world too: demons are only visible to humans if they want to be. But if a human gets a mashou- a certain kind of wound inflicted by a demon- they can see demons any time, regardless of whether the demon wants to be seen or not.

Fortunately it's pretty rare outside of controlled circumstances, usually new students at the academy have to have one purposefully inflicted on them to continue their studies. But it does happen sometimes.