From what I have read, the Smiths had been "together" 10 years, lived together for three but had been married less than a year. They were middle aged, had successful careers and each had children from prior marriages but no children together. Under the circumstances, it would be normal that they kept their finances separate and it would not be the least bit unusual if one (or both) had "secret" accounts or credit cards the other didn't know about. Judy may have financed her "trip" with one of these accounts. If she had a PO Box her husband didn't know about and a credit card in her maiden name, he and LE may never have learned of the charges. (It is also possible that he did find out about charges to an account for a lot of money; say for a rental car that was found abandoned that could easily run $5K to $10K, that he chose to conceal from LE for fear that he, as her husband, would be liable for the charges) This might explain his "reluctance" to take a Polygraph.
It is virtually impossible to rent a car without a credit card. (In the past some companies would accept a large cash deposit and a lot of good id). Either way, it seems unlikely, but not impossible that she rented a car and had it stolen by her assailant without LE ever becoming aware of it. I think it is far more likely she used public transportation and/or rode in a vehicle belonging to her new "friend".
One of the reported sightings in NC told of many bags of possessions in the car she was in. This would have been only a few days from her "disappearance" so it would be unlikely she would have obtained many processions that fast. If that was, in fact, her, the vehicle probably belonged to a traveling "companion" who was living a "nomadic" lifestyle.
The most likely explanation for her case is that she used the week she was expected to be in Philadelphia with her husband to take off on a fling/adventure/vacation from her marriage. She could expect her husband to be very upset but she may not have cared. Her children, family and friends would be another matter. Sometimes people just "pull a runner" and don't worry about the concern they cause their loved ones. She may have thought/hoped that her husband, out of shame, wouldn't tell anyone she was missing. If that was her expectation, she was wrong. These situations often raise the question of "mental illness".
If this "respectable, upper middle class woman really did just take off on a fling, is possible that she would hook-up with a "wanderer" with a car full of possessions? You wouldn't expect such a thing but, in this case, who knows.
Gary Hilton traveled around the Southern Appalachians during this period. I wonder if his whereabouts were ever established for April 1997.