Turns out my boss's boss, the VP of the division I work in, is also a runner and ran the half-marathon in the Asheville Citizen-Times race last weekend.
When he passed the "5K turn-off point," he commented to a fellow runner, "I bet a lot of people are going to miss that turn." There were very few volunteers at the turn-off, and the ones there were teenagers who wore jackets and other t-shirts over their volunteer t-shirts, and who more or less stood there and occasionally directed 5Kers to take the turn-off.
(No wonder I don't remember seeing any volunteers there.)
Then, as he proceeded along the half-marathon course, he saw several 5K runners backtracking after realizing they'd missed the turn.
"It was poorly marked," he said, and added that in previous years cones had been set up to "corral" the runners onto their respective routes. "I'm surprised more people didn't get off track this year."
So I feel better. The turn-off was poorly indicated. So I'll write a nice letter to the race organizers when I return my Champion Chip. And I'll check the map for good measure next year.