Visible crew/equipment: When they're all in the OR talking about Aggie being at the 4077, Charles compares Colonel Potter's art style to Aggie's, and in the next shot as Potter tells the art critic to tread softly, the boom mic is visible at the top, right side of the screen and just as the mic starts to pull up the shot zooms in on Aggie.
Visible crew/equipment: When BJ's in Margaret's tent talking to Aggie about feeling like a prisoner of war, just as he says, "I work because of the war," the boom mic dips into view at the top of the screen.
Answer: Even an Army field hospital would have SOME weapons for self-defense if the enemy attacked. What's meant is that it's not a combat outfit, and therefore wouldn't have enough weapons to actually attack anyone. (It's also mentioned a few times that the Officer of the Watch is supposed to carry a pistol at all times, but Hawkeye refuses to do it when he's in charge and Colonels Blake and Potter don't enforce the rule).
Captain Defenestrator