So... Second attempt at this mission. I'm almost done "cleaning" the three "office" maps. Happen to go in the meeting room...
The sequence of the High Overseer and Curnow talking outside starts. The door opens as if by magic. They quickly start several lines which are cut off by the next line starting prematurely. They (and the guards with them) act like they've seen something (not me!), and charge through the meeting room and out the other side! So I reload... Try again... Same thing happens. Try again... This time, I chase them, and by the time I'm killed I've followed them almost all the way to the entrance from the streets map. What's going on?!

I guess I'm going to have to keep trying old saves until I find one that doesn't do this. Ugh. Many hours wasted.

this game has some bugs with the rats...

I have had EVERY GUARD ON THE MAP run down to fight rats.

it might have been that. but what I did to fix it is I loaded an earlier save, like ten to twenty minutes earlier

Happened to me as well,had to restart the game.Im really becoming irked at bugs in Bethesda games.They are really out of control.

it is worth noting that campell just runs down to his safe room if there are any signs of a struggle, so You could just follow him...

Oh the rats... I'd seen that on the streets map... I go near the part with the rats in the low street, occasionally people will run from places they have no way of seeing me or the rats, just to kill them. Guards and thugs, who then fight each other...
I might go back to the entrance part and see if there's any rat swarms around there that I could get rid of.

Ah, I had wondered where he had gone, I saw Curnow but not him. That might be helpful as a last resort.

Well, I tried again... Used the last save I made before entering the office map, freed Martin, went straight to the meeting room, everything works as it should. Seems the drinks don't appear and the sequence doesn't run until you've freed Martin.
Using the first save I made in the office map (I had only dealt with two or so guards), and the weirdness happens!
I follow Curnow all the way to the exit to the brewery district, where he disappears. No rat swarms seen anywhere.

I finish the mission quickly to see what it says... 1 hostile kill (I killed Campbell), 1 dead/unconscious body discovered. As I have no idea where or when that was, it's pretty unhelpful. May have been what caused the craziness. I reloaded the save before entering the office map, quickly ran through killing Campbell and getting to the end. No bodies discovered, ghost. So the body must have been something I did during the 3 office maps. I really don't want to redo all that now though. Maybe I'll just wait for a patch, and by that time I'll be ready to do it all again.

Same thing happened to me: I just about finished cleaning the main building (didn't enter the kennels or the back yard), choking everyone out, unseen. Upon setting foot in the meeting room, I can hear the start of a conversation berween Campbell and Curnow, then the doors open and everyone freaks out and rushes out, swords drawn. Funny thing, I got the bodyguard achievement when this happened.

I know you may have tried this but this happened to me the first time so I re-loaded and this time shut the door behind me after I entered the meeting room and then hid behind the screen to the left of the doors that they enter through (the first time I had left the dorr I etered through open) and the second time they walked in and I was able to sleep dart both, carry Campbell to the Interrogation room and brand him and then carry the other guy to saftey.

I dont know if I was just lucky but it certainly worked for me

This happened to me when guards had discovered an unconscious body. I reloaded and hid them more carefully and the scene proceeded as normal. However, Campbell and Curnow also detect you really fast if you move when they come out. Gotta be speedy with whatever it is you're doing.

BTW can we please not just assume everything's a bug just because you don't understand what's happening? I haven't had a single glitch or bug in 1 3/4 playthroughs.