Recently, I posted a puzzle on Puzzling.SE.
The puzzle describes a prison containing thousands of hallways which all run next to each other, like the pipes in a pan flute. In my description of the prison, I wrote that most of the hallways have a door which leads to the adjacent hallway.
Someone wrote in the comments that my use of the word "the" is incorrect. After all, most of the hallways have two adjacent hallways, and so I can't write "the adjacent hallway"; I have to write "an adjacent hallway" or "one of the adjacent hallways".
However, I could swear that it's reasonably common in English to write things like "the adjacent room" and "the adjacent lot" even when there are multiple adjacent rooms and multiple adjacent lots, and no way for the reader to determine exactly which room or lot was meant.
I could very easily imagine someone saying "I was lecturing once when I heard a strange noise from the adjacent classroom", even when there are two adjacent classrooms; and "there are trees on the line between this lot and the adjacent lot", even when there are several adjacent lots. To me, if you replace "the" with "an" in either of those examples, it sounds bit like you're trying to draw the reader's attention to the existence of a new classroom or lot that they weren't aware of before.
Is my thinking correct? Is it really appropriate to write "the adjacent room" when there are several adjacent rooms, or am I just imagining things?
I'm interested primarily in whether or not "the" is actually used this way by careful writers, not in what textbooks for English students have to say about the matter.