Crime scene in Jessica's room

Axioms

The only Master keys are the ones held by the servants, one key each.
There are absolutely no types of hidden doors. This door is the only way in or out. The only way to lock this door is with Jessica's single key or the master keys, only one of which is held by each servant. It is on the second floor, but it is probably possible to enter or leave trough the window. However, the window is locked from the inside.
Kanon was killed in this room.
Of course, this door is the same as the chapel. When locked, it does not permit any form of entry. I'll say more. No trick could have the effect of locking the door from the outside without using a key.
Let's go back to Jessica's room. When Jessica's corpse was discovered, only Battler, George, Maria, Rosa, Genji, Gohda, Shannon, Kumasawa, and Nanjo were in Jessica's room. Whoops, the corpse of Jessica is also included. Therefore, both in the case involving Jessica's room and the one involving this servant room, no humans exist that you were not aware of. No one is hiding.
No method exists by which the door can be locked from the outside without using a key. Regarding the window, no method exists by which it could somehow be locked from the outside.

Corollaries

Sayo stabbed Jessica in the back

Kanon, aka. Sayo, betrays and kills Jessica in the second twilight of Turn. The magical battle represents her getting rid of Kanon's remorse and emotional baggage and metaphorically killing him.

For the details refer to Reading B in Theory for Turn of the Golden witch#4.1

Sayo was killed by X Beatrice.

The magical battle in the second twilight of Turn actually represents a conflict between X Beatrice and Sayo who isn't ready to attack Jessica and tries to defend her, ending in a double kill.

For the details refer to Reading A in Theory for Turn of the Golden witch#4.1

In both scenarios the culprits plus Rosa used a master key to set up the closed room and Rosa hid that fact.