To me, a Brit, a stand pipe is usually in a communal area (such as a street) with a tap attached, for the distribution of water during a period of time when the supply of water to a group of homes is cut off, or to control amounts used during a drought. It is connected to an underground supply pipe and stands erect, thus it's name.

Now, why you would find one behind a closet defeats me, unless the sign had simply ended up stored there - but yes, the sign itself is most definitely not a stand pipe, lol. I love the jokes on semiotics signs and photos can provide. They appeal to my pedantic streak!