At the FD, seen 'em mounted ceiling (combo units), at head level, chest level. and 16" off the floor (customary outlet level).
We normally tell folks to place them in their sleeping areas, and near, but not next to things that run automatically - like furnaces. Secondary considerations are dryers and stoves. I've seen elevated CO from both of them, but furnaces have the deadliest potential, due to cracked heat exchangers or such.
