Keen Senses is Chorizo's final Amigo ability: every so often he digs useful resources out of the ground for you. It rounds out his kit as a passive loot finder.
While you explore with Chorizo out, he sometimes stops and digs, turning up materials you can pick up. You do not have to trigger it, so it adds a trickle of resources over a long session.
It is a small bonus rather than a reason to pick him. Most players bring Chorizo for stealth, and Keen Senses is a nice extra on top.