Question:

Why is an Expected Document List (EDL) item updated by System without any trace in the Audit log?

Answer:

An update by the System occurs when the source of a lookup field is modified.

For example, on the EDL Item object, the Artifact ID is a lookup field to the Article ID field on the EDL Item Template object. If a customer modifies the source field, an update occurs on the other object record as well. Additionally, lookup field changes are not audited, this is why customers cannot see any record in the audit log. 

Related Documentation:

Vault Help Documentation: Workiing with EDLs