Waterways map spawning monsters in inaccessible location

Finished the regular league and am giving Ruthless a shot for the first time (awful, by the way).

I'm making sure to full clear maps both for drops and corpses. Repeatedly on the Waterways map, I will have full cleared it, uncovering all the fog, backtracking the full length of the map at least once, and it will still say there are double digit monsters remaining.

There is quite simply nowhere that is player accessible for monsters to be. I am including two images that show the count of 26 monsters remaining and the fully cleared map. I'd also like to point out that I'm using Skitterbots, which autotarget enemies and run up to them, and they didn't find anything either.