now, hope the community-office Stadtgemeindeamt Hallein
will clear up some for you. if your mother maybe went back directly to Hallein they must have them in their files at their Meldeamt/registry-office. otherwise i would hope you get info of other place she went there. we have now also a central registry in Vienna, where they can make a lookup online. because of this i thought you maybe send a scannning of a document which states your relationship. if they can find her adress, but will not tell you - think there would be the possibility,that they will ask her for you if she would like to contact you.
now in any case - good luck! :o)