Will those hidden walls be visible on the minimap from the safe area, or should I be wandering around randomly in the darkness flaring? I've never used a flare nor seen a hidden wall.
You can either venture off into the side corridors while you're following the cart to a node or go back down a corridor you've already lit up and venture off to the sides. You'll see an icon on your mini-map when you get close to a breakable wall. I recommend having 75% Dark Resist and 150% Light Radius for your flares at the depth you want to explore. The longer the distance between two points the better (higher chance for breakable walls).