Kay-VW
I hate it when people steal other peoples pets. I've had it happen before. A lady who used to live next door stole a bunch of my cats. I'm still mad at her because she took my favorite cat, Patches, along with her two kittens she had just had the week before >=(. I miss that cat, she was my baby.
Did you ever report her to the police? You could try to find out where she moved to by talking to the current neighbors and telling them your situation. Get them to ask the real estate agent if she told them where she was moving to, or if you're in an apartment, ask your landlord.
Jannyloujanjan: If you haven't done so already, go to where your cat was sighted and try to see if it's yours. If it isn't I might suggest to pick it up anyway and leave them at a shelter where they will be safe from the cold.