That Allied bombers routinely returned from enemy airspace with undropped bombs due to aborted missions, weather conditions, or tactical aborts is a matter of standard historical record and everyday observation regarding military aviation operations, requiring no specific citation.
The claim is a matter of well-documented military history regarding World War II air operations, but the provided papers discuss strategic bombing theory, ordnance detection, and historical models without directly studying or testing the routine return of unexpended bombs. Therefore, the claim is treated as common knowledge or a standard historical truism where no citation is strictly necessary.