A friend of my mine actually convinced the owner of the property to show her around and take her out to the pond that was drained. She was actually able to look through the area the pond was drained into to see if there was anything there of note. She posted a video on her Vimeo page of her walking through the area and checking out several features.
Based on this and the fact of the dogs going back and forth between the pond and the house I'm running down the thread of some other underground structure that Jason could have fallen into. Without any access to better aerial imagery, I have hired a drone pilot to go out and fly the roughly 160 acres around this area and capture some HD imagery. If this yields anything of interest then I'll go to the next step of doing a photogrammetry survey that can yield a sub 1 inch topographic map of the area. The pilot is heading out there Sunday morning so I will share the imagery here once I have it.