While there's a possibility they are or were in or near this canyon, it's hard for me to grasp the concept. One reason being obvious, they would have to cross the main road. To me this would be the main objective second to getting to the car. Then you sit and wait, or follow the road. The second problem is the canyon itself. While it's not entirely remote, the initial mouth of the canyon is wide and then narrows, getting steeper as you go. It's an elevation of 1400 ft. It would be foolhardy to attempt going. At that point they would have to be completely out of their minds.
I am thinking there could be an explanation:
Couple starts out on the easier Maze Loop trail. They complete the hike, then decide to attempt Smith Water Canyon on the same day. As they are not in trouble yet, crossing the main road does not faze them. As some point on the hike up the canyon, they get overwhelmed. Or, they get confused and hike into a canyon that they think is Smith Water canyon. They then get disoriented?
I think that is a good use of Occam's sharp razor. Another experienced hiker commenting on a Grand Canyon case also stated that National Parks are well patrolled and really don't have resident squatter colonies of the aggressively drunk / drugged nor the commercial pot growers that some National Forests or BLM lands do have.In a place where survival is dependent on great amounts of water and shelter from the environment. the least likely scenario is a ne'er-do-weller waiting out in the dangerous desert hoping a hiker will stumble upon them. It's possible sure, but it seems quite unlikely. Perhaps they did make it to a populated area and met with harm at that point. Again though, it seems unlikely.