The room where Chewie finds Threepio is probably the central location from which trash is sent from all over Cloud City to be sorted out, recycled or disintegrated.

I think the confusion over who shoots Threepio stems from the voice who asks "Who are you?". It sounds nothing like a stormtrooper, (I think it's Frank Oz in the original) to avoid tipping off the audience that the Empire is already there, potentially spoiling the big reveal of Vader in the dining room later. All we know at this point in the film is something smells in Cloud City, and it's not the Ugnaughts.

Doesn't Threepio say something about stormtroopers when Chewie reactivates him?