The folder of a backup plugin answers from outside.
Such a folder can hold a complete copy of your site including the database, with customer data and passwords. The file name often contains a random code, so an attacker has to guess, but that is not a safe place to keep it.
On WordPress sites Snaglist asks for the known folders of the common backup plugins, after first asking for a folder that certainly does not exist. A site that answers everything is not reported. It does not look inside the files.
In the report for your client this appears under the heading "Backup folders reachable from outside". When everything is in order, the report says: "No backup folders reachable from outside".
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