Well, the guy was a selfdeclared "merchant" and collector in all things connected to the third Reich. Neighbours had known about his tank at least since the 1970s, when he used his toy during the blizzard. Accordung to documents, he even had Bundeswehr specialists at his house on a regular basis to do repairs and maintenance to keep the tank in driving condition. The full extend of his "basement museum" came to light when federal investigators did a search of his house in connection with two statues from Hilters Reichskanzlei. The tank wasn´t even the highets priority for investigators. He also had a f**k ton of live ammunition and other weapons of war. Here´s a nice summary (in german): https://de.everybodywiki.com/Klaus-Dieter_Flick