Hnngh...

How did XX-sama deduce the truth from that alone?

There's one thing I didn't mention yet.

Huh? But Ringo, you said you called the office after you found the scene...

I knew about it by coincidence. Reisen and I had been giving a lecture from 2 to 3 PM.

The change was made at about 2 PM, and there'd been no warning of it.

From the lecture room window, I could see the electrical notice board in the courtyard, which had a message from the custodian.

Notice board?

That message kept going across the screen, so it stuck in my mind.

A primitive type of display based on an array of small lightbulbs. It can scroll at a speed of about 3 characters per second.

"NOTE: from 2 PM today, the custodian's office phone number will be 635619. Keep in mind"

They were actually doing some electrical work at the lab, and because of that, the custodian's phone number had been abruptly changed.