I recall seeing that in the editor and I'm fairly certain from the condition of it and glancing over the linedefs down there that it was an area later scrapped after being designed, which was not removed to avoid changing the line numbers of content made after it, which, due to the way the scripting works, is a very compelling reason - you change lower numbered linedefs much and you have to redo every line reference in the scripts for things like switch textures. Not pretty.
Of probably greater interest is a secret pillar with stuff in it in I think, Q3M2 or so. It won't open in 1.0 because the light fixture inside overlaps the pillar wall and makes it decide it's blocked and can't move. Dunno if that got fixed in 1.04 or not because I haven't gotten back there yet, but I doubt it. Another one for the 1.05 TODO list, probably, perhaps along with actually re-implementing the secret area this topic is about.