The ones I have seen (older, roll-up door Costco's) are low on the wall, 2'-6" to 3'-0" off the ground.
They are usually placed between the entrance and exit doors, closer to the entrance door.
There is also usually steel poles coming out of the ground, the sign cannot be seen from certain angles because of this pole.
Also if you are entering when others are entering, there is a real good chance someone will be blocking the sign.
Their placement of the sign is not in plain view, IMO.