the girls wouldn't have kept their stage wardrobe in their hotel rooms. those gowns, earrings and accessories [[including their stage wigs) would have been kept in the dressing room. D and C were robbed of their personal possessions. So these purple chiffon gowns wouldn't have been stolen.

but you're possibly quite right that these were worn for many months and yet seem to have slipped by without much notice. the girls were performing constantly. tours, one nighters, big multi-week engagements at the big supper clubs. I'd guess they traveled with 6 - 8 sets of gowns at any one time. the wardrobe crew would have handle the logistics and gowns would have been rotated in and out - off to the cleaners, to tailors for repairs, back to the storage facilities the group would have used..