I just assumed they couldn't be seen. I ran it twice and the second time I was hurrying the run and not immersing myself so I wasn't super careful and just brushed past the patrol routes without giving a second thought to how tight the timing would be for NPCs behind me and I had no issue.

I actually loved that duty. My only gripe with it is how hopeless they seem to think the players are and therefore trivialise the difficulty instead of making it a little bit more fun and interesting. I'm not expecting Tomb Raider-tier puzzles or anything that take up much of your time, but something that makes you stop and think would be great for sure.