A building says "you are here" upside down over it's doorway.
The building's sign is upside down.
An object with a message (such as "you are here") on it is known as a sign.  Since the sign is located over the building's doorway, it is reasonable to assume that it is the building's sign.  Wording on signs is generally upright; therefore, if the message on the sign is upside down, the sign itself is most likely upside down.
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