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It would be easy enough to add a flashback scene or just refer to the cleaning of Grimmauld Place at the appropriate time. They could also skip the cleaning entirely and just have Mundungus or Kreacher steal it directly.

One of the few times we see Kreature he's putting something in a box and lugging it away.

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One of the few times we see Kreature he's putting something in a box and lugging it away.

The box looks like an old open fruit crate. He is clearly messing around with something just out of the field of view then you hear a tinkling as it looks like he puts it in the box. he walks right under Harry's nose and inverts the now clearly empty box to stand on it to polish his mistress' portrait's plaque.