Interesting point, TF. If they set off together after pre-drinks, someone would presumably have a memory of her top or even a photo (she was hugging everyone, we know it was a big deal that they were all back together, therefore a natural time to take group photos).
Top wouldn't be relevant to any living street sightings as it was so cold. Yet highly relevant if it was found discarded in the park, say.
Could suggest she popped home to change, then only met her pals after she had her jacket on, so noone knows what she had on?
(I have undergone missing persons enquiries several times with one of my own family members - the police asked us to go through the wardrobe to try to ID any clothes missing.)
They said originally that she had a long sleeved black top on.