People present on the fourth floor did not know that some people were entering the fourth floor. All the teachers present on the fourth floor are currently busy observing those students. They need to stay ready to help any of those students at any time.
On the other hand, those people with cat masks have already entered the fourth floor. Everyone has an artifact present in their body which also helps them to hide themselves.
If you look at those people then you will see they have some kind of locket-like thing present in their neck. If you focus on that locket then you will notice it is a skull-like thing. The eyes of that skull glowed purple color.
That is one of the dark artifacts that help those people to hide their presence. Naturally, there is a chance that those people got found out by those guardians present in the place, but they still did not fear any of those Guardians. All those people with cat masks are ready to die anytime.