the walk built so Hal can slip away — "Mario could sense that Hal wanted…"
Mario could sense that Hal wanted to be able to go off by himself briefly, so he contrived (Mario did) to be very interested in some sort of leaf-and-twig ensemble off the path, and let Hal sort of melt away down the path.
Context
The post-prandial stroll exists as scaffolding for the using — Hal with his gear bag, a route to the tree-line — and the tenderness of the scene is that Mario knows it, and covers for it, feigning interest in a leaf-and-twig ensemble so his brother can melt away unremarked. The enabling is done out of love and tact, which makes it a quiet system all its own, indistinguishable from care.