Almost like...

they were actually trying to stretch out the call?

Indeed, the second contradiction lies within the call too. Why didn't the culprit only ask to open the lock on the back door?

The culprit is calling to have someone unlock the doors leading outside the lab.

But in that case, the boiler room exit they specifically locked would be opened as well.

Unlock

But they were pretending to be a visiting lecturer, right?

Unlock

No. When the culprit was asked which storeroom they were in,

Ah......

Wasn't that part of using the teaching materials as an excuse?

South back door

Storeroom

they could've just said they were in the storeroom next to the night duty room.

If they said they were taking equipment from there, they would've only needed to open the back door.

A culprit who wanted to quickly escape unseen had literally no reason to open the boiler room exit as well.

And if they were calling from the night duty room's internal phone, there's no way they wouldn't have known about this room.