Horror stories?
The best reason for a 3-4 year old car to be on the lot at a dealer with under 10k miles a year on it is because it was out of lease. There was some paperwork mixed in with the original owners manual in the glove department and I figured out that the leasse worked 3 miles from home, therefore driving some 7k miles a year. It was still under warranty when the electrical started acting up as the car warmed up, (It wasn't being driven far enough when the lease ended to notice the problem) there was a bit of corrosion on a wiring connection and it was replaced at no cost.
The WORST reason for a car having low mileage is that it was wrecked, in a flood, etc. So it's off the road for awhile. You might notice the paint looks great in the front but they were too cheap to paint the whole thing and note the wear and tear on the finish as you get to the back of the car. Sometimes miscellaneous parts don't hold up well after the wreck. It might not be a lemon, but nagging things like a belt tensioner, steering idler arm, etc. If there's bad ball joints in the steering you might not readily notice but it could affect stability in situations.
Not so much today but in the past you'd think around 100,000 miles you'd need a water pump, maybe a fuel pump, etc. Then again around 200,000. Which is why people get touchy around such mileage, you get it home and in the next year or two if you don't work on it yourself it can total up to thousands for the multiple times to take it to the shop, maybe have it towed, etc.
In California there's the issue of passing smog. You'll have to do it ever 2 years, but you don't get the smog report from the dealer when you buy the car, you don't know how close it is. And if a sensor goes out, another potential 100k mile problem, you have to replace it before it's smogged. I can do some of them easy enough, but say a crankshaft position sensor is a major dismantle to get to. If you see so much as a quick flash of any light on that dashboard you don't want the thing.