NFPA 10: Standard for Portable Fire Extinguishers, 2013 Edition:
6.1.3.3 Visual Obstructions.
6.1.3.3.1 Fire extinguishers shall not be obstructed or obscured from view.
6.1.3.3.2 In large rooms and in certain locations where visual obstructions cannot be completely avoided, means shall be provided to indicate the extinguisher location.
6.1.3.3.3 Where signs are used to indicate fire extinguisher location, the signs shall comply with the following:
(1) They shall be located in close proximity to the extinguisher.
(2) They shall be visible from the normal path of travel.