Are there mirrors on a 135 that provide a rearward view? Seems most helicopter mirrors I’ve seen provide a downward view for landing or sling ops.
Mirrors are an optional installation. However on this bird, the mounting areas where mirrors would go, appear to be taken up with the Night Sun light installation.
Even so, with their turn they made back to the airport, I do wonder if the back seat crewman thought to take a look aft inside the turn, for any smoke. If they only suspected a failure and didn’t have indications of a fire, they may not have thought to do so.