I don't think it's at all certain that his body would have been found by now. If he wandered in the wrong direction while confused, fell down an embankment while trying to reach help, or was struck and flipped by a car, the body could have fallen into a ditch, swampy patch, body of water, etc. If he was in an area people didn't expect him to be in, he'd be even less likely to be discovered. Water was at record high levels. He could have stumbled into a stream where it's usually dry and been washed quite a distance.
I'm thinking that if he did have something private, he might have told a bit of a white lie about the trip -- that not all the long trip was caused by the flooding, but it made a good excuse. And if he did find a topographical map to get a better route, that would shorten the trip.
I dont see a flood related incident here. Either Brian and the car would have been stuck in the water or the car would have gotten stuck in water and Brian would have been washed away. I don't think a flood damaged vehicle would have been drivable to Chattanooga.