But during maintenance, the board was lowered to the ground and in the shadow of a fence. You couldn't see it from the hallway.

The time it was raised back up was the exact same moment we were bringing in the whiteboard.

In other words... The only option is that the culprit saw the message in the short gap between us entering the hallway and the murder being committed.

B-But there's no way to see the courtyard from the boiler room......

...AH!!

......Right.

There is a way.

I left the door at about a 45° angle.

From this angle, the notice board would be reflected on the mirror on the storeroom door.

Notice board

Door

Culprit

On the mirror, the reversed letters would've been flipped back to normal again, making them easy to read.

However, only a small part of the board is visible from this direction.

Time when the culprit could've seen the notice board from the hallway

- Reisen heads to boiler room - Ringo props storeroom door open - Boiler room door opens

- Reisen screams

Despite that, the culprit was able to read the message scrolling at only 3 characters per second and call the custodian.

Which means—