This is written by one of my colleagues and I don't think it sounds quite right:
please pop by to Room 159.
Shouldn't it be re-written as:
Please pop by at room 159?
I would always write or say, "Please pop by room 159", and I'm sure that's the form I mostly hear.
I would reserve "at" for, "Please pop by at 2pm today", "Please pop by at your convenience" etc.
not a teacher