I first suddenly realised that mum had dementia when she, out of the blue, phoned up and told me that an old and very dear friend of hers was stealing from her. Of course this friend was not stealing anything - mum was moving things around and forgetting where she had put them, so thought someone was stealing things, so she would "put things away safely" (aka - hiding them) so that no-one would find them and steal them - except, of course, she couldnt remember doing it, so it became a vicious circle.
This accusation is so common it is almost diagnostic, so Im very surprised that SS say its not dementia - it most certainly is!