I live in New Orleans part-time and evacuated the city a couple days prior to Katrina. Before I left the city, I locked my home’s doors and checked that the doors of the apartment were locked.
When I returned the front of my house and every other house in the city had spray painted markings left by Guard indicating what was found in the house. There were markings for dead people, live people and animals dead or alive.
And, the doors were unlocked and nothing was taken.
So yes, all houses were entered but they did not attempt to confiscate all guns.