i would speak to the police as it is indeed theft.
cats are largely independant animals and can roam around for days, going wherever they like, without being seen by their owners - this does not mean they do not have an owner.
for this woman to have actually caught your cat and kept it and asked around because she genuinely thought it was abandoned sounds unlikely
assuming the cat was well fed there is no reason to suspect that any cat you see has no home.
did the cat came to her for food and she fed it? - as many cats do, they are scavengers and will take food where they can get it - that again does not mean it has no owner - but i suppose you can understand if she took that behaviour to mean that the cat was not getting fed elsewhere, but the fact that a new cat appears at the same time you did - and she didn't ask you implies she perhaps knew the cat was yours but simply does not like cats and wanted rid of it, so cooked up this scheme to get rid of the cat.
does she have pets?
i would tell her you are going to get 4 new cats to heal your broken heart - watch her reaction - and also you expect her to pay for one of them as she is responsible for your loss.
mentioning the police will undoubtedly make her and the sanctuary think again.
the sooner they act the better as i would hate to think of some kids getting attached to their new pet only to have it taken away again.



