Darn; I thought I had found all of those before release, but obviously not. Because of the way the grid is set up, whenever walkable tiles are adjacent to each other I have to manually mark those edges as walls (or make them too high to climb, which only works in one direction). Do you happen to remember where it was? I did just find one next to the second upgrade station (fortunately that one only goes backwards, so no skipping ahead there).
Really glad that you enjoyed the game otherwise, thanks for playing!