oh i see whats going on. i found the event by accident when i figured out how to enter the other side of the block, but i was going all around the highway to update the article at the same time. the shadow monster event switch is linked to the generic looping part of highway that gets reused on all the roads.
tldr; if you go across all the blank highway sections that are just the road with no other features, one of them triggers it. ill edit the page and fix it once i figure out which one it is.