It is probably that an unconscious hostile got eaten by rats.

I am also experiencing the same issue on the same mission as you, that although I think I made no executions, there is one hostile death count.

After tried for about five times, I successed with no death count. What have been changed is that I deliberately chose not to chock the ninja in Baud's headquarter.

There are rats in Baud's headquarter, so if you put down ninjas unconscious, the ninjas might get eaten by the rats, counting as your hostile death count.

It is probably that an unconscious hostile got eaten by rats.

I am also experiencing the same issue on the same mission as you, that although I think I made no executions, there is one hostile death count.

After tried for about five times, I successed with no death count. What have been changed is that I deliberately chose not to chock the ninja in Baud's headquarter.

There are rats in Baud's headquarter, so if you put down ninjas unconscious, the ninjas might get eaten by the rats, counting as your hostile death count.

Update: I think in my case it's not the problem of the Granny and Slackjaw, since that

in my playthrough I haven't met them, possibly because I didn't conducted some certain side-missions.