I'm working on "dead eels" mission and have run through it twice and been unable to get 'undetected' both times. Are there some quirky/buggy things that can result in this? I never hear any detection and am super careful with placing guys I've knocked out in safe spots.

I go through the mission roughly this way:

Get to the ship, dive under and enter through the bottom to knock out Wakefield. This is probably the only body that I don't put in a super safe spot, I just tuck him into the corner out of view of the other crew. I then exit through the bottom of the ship and work my way around to do the horn from the outside.

next portion:

Head over to the clothing factory and get the info on the guy in the sewer. I also grab the sewer key from the nurse with "pull", but don't enter. (still ends up triggering the "you found the geezer" update when I open the door, so I'm wondering also if this could be doing it)

Next portion:

get into the sewers and get close to the body and pull the code through the bars rather than wind around and mess with the witches...although I did sleep dart the witch on the raised platform, but stayed hidden to do it and never triggered her speaking. I do get seen by the spitter plants, but I don't think they matter.

Next portion:

Headed back to factory and head down to get the engine part and then take it to the ship. The witches trigger when I put it in the slot, but I'm pretty sure that's an unavoidable detection, correct?

Anyway, any advice would be appreciated.

Hey, I'll leave it up in case it helps someone else, but it turned out to be the witches at the end that were triggered by placing the engine part. I warped up immediately to a higher point on the ship after placing it and used the "statuesque" rune and let the crew handle the witches. Got "undetected" no problem.

Thank you, thank you, thank you for this! I completed that mission for the third time last night and still no undetected... Thanks to your thread though I reloaded a save and finally got it.

Only problem is, that even though I got undetected in the final mission already, the game didn't recognise that I had undetected for all missions so the Silence is Golden trophy didn't unlock. So I have to redo the last mission for the trophy.
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Yep, that's what I figured too, which really blows because 1) I didn't know I was detected and 2) found this out by starting the engine (I didn't kill any of the witches). When this happened, it goes to my tally screen and what's worse, the auto-save ruined the chance of trying to figure this out. My last save was 40 minutes back.

I mean, seriously: who the hell would think to stop time, place the ship's core, then zip out of the way??!! Bad mojo, Arkane. Boo!

PS: Statuesque does not work if you're in the line of sight. Learned this the hard way.