Flying the US Flag upside down has always indicated a sign of distress, and I believe it is so for all countries that have the intelligence to issue a set of rules/regulations for flying their flags. Throughout history, it has been noted that ships will fly the flag upside down in distress if they are sinking; provided they have time to turn the flag upside down before the ship goes down.
If I saw a Post Office flying a flag upside down, and no one was going postal, I would contact the local postal inspector immediately, take a photo, and have the idiot fired who raised the flag.
From the source:
UNITED STATES CODE, TITLE 4--FLAG AND SEAL, SEAT OF GOVERNMENT, AND THE STATES, CHAPTER ONE-THE FLAG, SEC 8. RESPECT FOR FLAG.
No disrespect should be shown to the flag of the United States of
America; the flag should not be dipped to any person or thing.
Regimental colors, State flags, and organization or institutional flags
are to be dipped as a mark of honor.
(a) The flag should never be displayed with the union down, except as a signal of dire distress in instances of extreme danger to life or property.
Those idiot kids who flew the flag upside down, with a Mexican flag on top of the pole, a few weeks ago during the demonstrations, did not know, or didn't care, that it is actually a sign of distress. Then again, these people actually think that CA belongs to Mexico.