A wrong turn on a dark road can lead to driving directly into some lakes because of poorly sited and poorly marked boat ramps,as well as large ponds and streams near lightly travelled roads.Only in a few cases is the water so transparent that a car submerged a few feet beneath the surface would be noticed.Several persons have died mistakenly driving into one Indiana (Brookville?)lake via the boat ramp after dark.
As for criminal activity,cars can be hidden in barns,garages,or old factories and taken apart or shipped out in a box truck or hauled off in one of the thousands of enclosed trailers seen every day.Cars can even be buried;a goofy neighbor buried his decrepit station wagon years ago rather than just calling up the junkyard.He thought he would have to pay to have it removed,but he did have a company back-hoe sitting there ,so.......
Since no credit cards use has shown up in all this time,I have to lean towards the "accidentally drove into water or ravine" theory.Of course the police will also be watching the relatives in regards to how they act,life insurance claims and so on.Sadly,a number of missing persons cases are disguised killings by relatives.
As a society,it is odd how little checking is done of,and how low the wages are, of persons who clean our factories and medical institutes,give assistive care to the eldery and handicapped,and also watch our factories for unwanted activities.Too many people think such work is not important until it is their property or person being cared for,yet still few are willing to spend the money for screening and pay.