Vienna Government Employee at my door

At 7:00 am this morning, I had someone banging on my door. Turned out it was the same guy who put his name on a folded piece of paper on my door yesterday. First he flashed some kind of badge at me and then rudely demand to know why I had not called him back as he tried to walk into my apartment. I closed my door behind me so he could not push his way in, and had to tell him twice that his office hours were over by the time I reached home from my job the day before. He proceeded to ask me for my name, my phone number, how long I have lived there, etc, all to which I was not really willing to answer since I had no idea what was going on, and I don't care what badge he flashes, I live on my own and not about to let someone I have never met before push his way into my apartment, and also take all my personal information.
He explained that he is looking for some guy, who apparently still had his melzedelzhung (or however this is spelled) at my residence, and I told him this person has not lived here for years, and that if he wanted my information, the Austrian government had it (and I gave him my last name). At the end I also gave him my phone number, and he was telling me he would have to send over the police to get my first name, which of course sounds crazy to me when the Austrian government, who he claims to work for, has all my information.... I also found out that he already talked to all my neighbors yesterday to ask me how long I have lived here...
Eventually he gave me his business card. It says
Magistrat der Stadt Wien
Magistratsabteilung 6, Rechnungs und Abgabenwesen, Erhebungs, and blacked out Vollstreckungsdienst
Anyone know anything about this?