I wonder if Charles parked when he picked Theresa up at the airport? Or did she text/call him and he picked her up curbside? If he parked, he would have to pay to park, or walk a mile and across a highway for the free parking. You take a ticket at a gate when you park, even if it's late at night, and there isn't an attendant. There's a bar that's down until you take it. Once you leave, there's another gate you leave, and you feed the ticket into a machine which reads it. The bar doesn't come up unless you pay. How much you pay depends on how long you've been parked. It can get pretty expensive. After a certain time of night, there isn't an attendant. There are many parking lots, some are covered, others are not. Regardless, there has to be plenty of cameras around the airport. It wouldn't surprise me if LE has asked for the footage. It also wouldn't surprise me if there are cameras at the parking gates too. Maybe they could see whether there was an extra "passenger" with them. I'm not believing that they were carjacked/kidnapped, but it's a possibility.
Why they would hang a right and drive a fair distance to the middle of no where, there are several possibilities. Bathroom break, intimate time, a place to sleep away from the highway are the more harmless ones. Obviously didn't end that way. Could've driven there to leave her body, thinking it would take awhile for it to be discovered. Except harvest is still going on here in Whitman County. Wheat is done, but there are still garbanzo beans to be harvested. Not sure what goes on in Lincoln County as far as crops, but there are plenty of farmers out and about at this time of year, so leaving her where she was found was pretty stupid. Thankfully.