When you query for an AV signature name or Signature ID in Threatvault, the sha256 hashes reported in Threatvault are the sha256 hashes of files for which this signature was generated.
e.g. If you search for the following:
Name: Virus/Win32.WGeneric.brwesg Sig id: 438164772
Threatvault shows 318f37ae7d23f954c51f3aef2d06edaaec60e01639131f2efb801cbab7168fec as one of the sample SHA256 hashes for this signature. This sample 318f3.. was deemed to be malware in the WildFire sandbox and thus the signature shows the association as coverage for this sample.
However, if the file transfer that triggered a signature was not for one of the hashes listed or no other malware was found, as @OtakarKlier suggested, please report this via a case and a support expert can help with next steps.
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