Ask who knows what directly
A speaker may accurately report what another person told them while having no direct knowledge of the underlying event. Preserve that distinction. The statement I heard he left at dawn establishes a report of a departure claim, not necessarily the departure itself. The next useful question is who supplied the information and whether the file contains that person's account.
Worked example: the closed gate
An interview says the gate was locked because a colleague mentioned being unable to enter. The colleague may have tried a different gate or arrived outside the stated hours. Reading the original message could narrow the claim. The secondhand account remains useful as evidence of what the interviewee believed, even if it does not settle the physical state of the gate.
Map the chain briefly
Write observer, first report and later repetition as separate steps where the case supports them. Avoid inventing a missing source to make the chain complete. In Somatry, this prevents several repetitions from becoming independent corroboration and helps distinguish a character's knowledge from the event itself. Someone acting on a mistaken report may still be described accurately by an interview about what they were told.