Hnngh...

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

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

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

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

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.

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

Notice board?

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"

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