I posted this in the poxburster thread, but its all related. My clip shows two trappers approaching silently, too.
Audio is jacked up. Based on dev comments in that thread sounds are ranked by importance, and sounds the game deems ‘not important’ get muted or their volume is turned down.
So next time you get smacked from behind but never heard anything approaching, you probably didn’t miss it. The game decided it wasn’t important and didn’t alert you.