Why were the de Kuypers stolen?


Hint 1 of 6:
A lot of people were kept out of the auction because of the timing.

Hint 2 of 6:
The paintings were very suspicious.

Hint 3 of 6:
The paint on them was very new.

Hint 4 of 6:
In fact, the paint was made after De Kuyper's death.

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