I have my little non paranormal theory. Keeping in mind that I have no exposure to the case of these missing people except this movie and the last one.
My theory is that the people are being hunted by other people - people with money.
Dogs can't find scent because the hunted are being targeted near water, to cover scent - that could be planned.
A theme that was mentioned about the people were that they usually had a disability/illness. They could prefer these people for the hunt so they don't get caught.
The cases in the film mention that that even though the hunted were older they were experienced hunters or fit or they knew the area - so they likely visited the locations multiple times and were known to the area, which could explain why they were targeted by the hunters in this theory.
Animals were ruled out because the hunted's equipment was usually moved or disappeared.
Why specifically people with money?
Hunting is a common pastime for the affluent. Money equals the means to be able to take time to plan hunts. It could explain how a cover up may have happened. One of the cases mentioned sudden fog for example - that could be man made fog so expensive equipment may be involved to plan hunts. Part of the hunt would be the getting away with it or the hiding it from suspicion. Regular serial killers would be ruled out because they usually kill for infamy or what their deeds to be known, or they kill to interact with the body in ~some way~.
There are multiple movies made about people with money hunting "regular people" for fun...
This probably won't fit all of the cases though.