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The article seems to state there wasn't a lot of value in the cold wallets.

Yes: "Ernst and Young had managed to identify six Quadriga cold wallets so far but had found almost nothing inside any of them."

But could it be possible that there exist other unknown cold wallets with a lot of value with lost passwords?

It seems more likely that there exist other wallets with a lot of value, whose password is known by their living owner, Mr Cotten.