so we could see it from the lecture room.

The board had been lowered to the ground for maintenance

Lecture room

So in that case, the culprit left the boiler room and stopped by the night duty room on their way out...?

Courtyard

Notice board

Window

Window

We can assume that this is how the culprit knew the number.

To stop any research from being stolen, the system is designed to make escaping harder than getting in, which is why the culprit was forced to call the custodian.

Could they have gotten in through a courtyard window?

Ah, no no.

The lab is underground, you see. It's not actually open on top, so there's no way in or out through the courtyard.

Main entrance

West back door

South back door

East back door

Ah! That spot could also be seen from the night duty room!

There's also the question of how they got in. The second floor of the lab building has high-class sensors to stop a break-in, but the first floor with the boiler room is pretty lightly protected. A youkai who can hide themselves or change their shape would have no trouble getting inside.

However, all the external doors have automatic locks on the inside and won't open without a key.