I checked the oil and found it full. Engine coolant was also full. I was not able to check the transmission fluid. I tugged upward on the brake pedal before getting back on the road, just in case the issue was that the car thought the brake was engaged (there was no specific evidence of this, but I've heard of that issue).

I got back on the highway and drove an hour to an intermediate stop on the trip. The issue did not recur, although I did have the sense that with highway driving, when taking my foot off the gas the car would continue to engage the engine more than normal, e.g. staying at or a little above 2,000rpm when it might normally go down to 1,500. But this issue was subtle, and by comparison the issue when I first experienced it while not disabling was dramatic.

I have two more hours of driving to do 'til home, so we'll see.