Are there any unique reasons for hostile deaths in mission 7?

A few weeks ago I started a play through on hard difficulty especially for the clean hands achievement. I got all missions except 7 (The Flooded District) with clean hands at first try (also later ones).

I have tried at least three times to do 7 and there always is one hostile dead (in one case even more). I payed special attention to the following:

• Granny and Slackjaw. I tried all the following. Only stealing the key and running away to the drainage, Letting Slackjaw kill her,Choking her.
• I used no means of killing throughout the mission, except one grenade on river crusts and some bolts on river crusts. I used no sleep darts for fear of anyone falling from heights or into fluids.
• I tested again and again, whether the unconscious bodies of assassins remained on roofs where I placed them.
• I defeated and spared daud. I didn't attack his lackeys during the fight (except maybe the first play through, where I was startled)
• I carried all weepers near my gear away from the floor, to protect them from rats.

Naturally, nobody can tell me my mistake, but perhaps you remember, whether there is anything unique to watch out for in this mission.