In Dishonored, leaving bodies lying around, even unconscious ones, can cause several issues.
First of all, if another character walks by, they will be alarmed and start looking for you which is never good.
More importantly though, if you leave an unconscious body out in the open that you don’t intend to kill, if you’re trying to reduce your chaos level or get the Clean Hands achievement / trophy, then you run into the possibility of rats coming along and eating it which does count as a kill.
If you want to hide bodies completely so that they’re not found by anyone, and so that rats cant’ get to them, then you need to put them into the conveniently located trash bins scattered around the map.
Alternatively if you’re just trying to keep rats from finding the body and you don’t care if it is found by other characters, just make sure to either place the body into a room with a door that you can shut that doesn’t have rat holes in it (just to be safe, I’m not sure if the rat packs use these or not), or up on something raised off of the ground that the rats can’t reach.
From what I’ve read, bodies found don’t count against your Ghost status anyway, so this will be the easier method in the long run than running to a trash bin every time you knock someone out.