Guns left overnight or longer are in the custody of the FFL in whose bound book they are recorded. The FFL cannot allow them to leave the premises in the possession of someone else without doing a transfer.
If the gunsmith has possession of the firearm from one business day to another or longer, the firearm must be recorded as an “acquisition” and a “disposition” in the permanent "bound book" record.
Q: Is an ATF Form 4473 required when a gunsmith returns a repaired firearm?
No, provided the firearm is returned to the person from whom it was received.
[27 CFR 478.124(a)]
This part covers my experience- no transfer, even if held for several days.