Hello,
I'm a bit confused by the Anton Sokolov level which I've just finished. For every guard on the way to Sokolov I used non-lethal (knocked unconscious usually from sneak from behind) and hid them (pressing X not throw when dropping them). I think I only killed one as I dropped him from the bridge by mistake

In the actual laboratory I did the same and put them all on the pipes around the edge of the building. I also released all the citizens locked up that I found too.

But when I finished the game it said I killed 7 and did only 4 non-lethal and gave me a high chaos.

Did I do something wrong with knocking them unconscious or hiding them and did it ignore being nice and freeing everyone? I can't remember the chaos for the other missions but I've been good all the way through...

The chaos is the overall chaos from all the previous missions, so if you had high chaos before this mission you might still be high chaos because of that.

Also, if you leave unconcious people on the ground, and they later die (eaten by rats or whatever), it still counts as a kill for you.

ok, I guess having an in-game count on one of the options pages would be handy.

I didn't know about being eaten by rats though, seems a bit harsh. But please don't say stacking guards on top of each other kills them through suffocation too

Doubt it, but you can kill unconscious guards by throwing into the water, or merely dropping them from too high to the ground. Maybe try to release them with [F] instead of mouse buttons, it provides a softer landing.