Re: Why now?
If one reads the german article there are connections between the owner and (quote from the SZ)
"Im Zuge der Ermittlungen um nach Jahrzehnten wieder aufgetauchte Nazi-Kunst waren die Behörden auf die Spur des Norddeutschen gelangt."
So the initial investigations where along re-apperaing art from the 3. Reich (most likely stolen art) and then the officials found the tank, an 88mm Anti Aircraft gun, a 533mm Torpedo and lots of other weapons. They have been confiscated and will be checked if they are properly disabled/disarmed. Until then they are removed to an official place for lockdown to prevent illegal use if NOT disarmed.
A 75L70 still is a VERY potent gun, punching through anything short of a modern tank after all. (The 88L56 AA gun actually is less capabel in that job)