Some teachers don't have their own classrooms (there are not enough rooms for each teacher to have one). They have to live like gypsies, packing up and moving to a new room each period. They use the classroom of a teacher who has a prep period that period.
Today I had an interesting synchronicity of events. I was asked to cover an extra period, and the teacher I was to cover used the classroom that I was already in.
Some things are so common that I rarely comment upon them. But some things never happen. You'd think that with the prevalence of covering extra periods and roving teachers this would happen more frequently, but it does not.