Visible crew/equipment: After Klinger gives the envelope with the forged discharge papers to the driver going to I Corps, just as he walks through the doorway we can see the two actors' tape marks on the ground, where the driver and Sidney were standing when they arrived.
Visible crew/equipment: When Sidney arrives and speaks with Klinger in the office the door remains open for the next few shots, and while Klinger first talks to the doctor and then to the driver, we can see the top of the crooked "sky" backdrop behind the tent in the compound.
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