I think they mean that the assets do not appear on the books.
Seehttps://www.accountingtoday.com/news/unveiling-the-mystery-o...
"We identified over 250 fictitious entities used to purchase properties and ultimately obfuscate assets from the trustee. Profits from the sale of the properties were concealed under the guise of substitute purchasers and also through hundreds of fictitious bank, brokerage and other accounts. "

see:

"The disposal of assets involves eliminating assets from the accounting records. This is needed to completely remove all traces of an asset from the balance sheet (known as derecognition). An asset disposal may require the recording of a gain or loss on the transaction in the reporting period when the disposal occurs."

(https://www.accountingtools.com/articles/how-do-i-record-the... )

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Note added at 10 mins (2020-10-29 08:43:54 GMT)
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"ADVANCED BOOKKEEPING
KAPLAN PUBLISHING
Disposal of capital assets
Introduction

When a capital asset or non-current asset is disposed of there are a
variety of accounting calculations and entries that need to be made.

Firstly, the asset being disposed of must be removed from the accounting
records as it is no longer controlled. In most cases the asset will be
disposed of for either more or less than its carrying value leading to a
profit or a loss on disposal which must be accounted for.

Finally, the fact that the asset has been disposed of must be recorded in
the non-current asset register.

In most examinations you will be required to put through the accounting
entries for the disposal of a capital asset (i.e. a non-current asset) and to
record the disposal in the non-current asset register."

(https://kaplan.co.uk/docs/default-source/pdfs/study-options-... )

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Note added at 12 mins (2020-10-29 08:45:34 GMT)
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Juridisch-Economisch Lexicon (JurLex):
wegmaking =
being disposed of
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Concealment of Belgian assets.