"I read some previous threads onbrickboard about the coolant light coming on due to a faulty/dirty sensor in the reservoir, but that doesn't seem to be my problem."

How do you know that it is not your problem.

From the way you wrote your post it sounds like you suspected that the sensor might be the problem, but you took it to the dealer, who ran some diagnostics, which came up clean, so you've decided the sensor is good.

If this was your thinking, it is wrong.

Just because the diagnostics came up clean does not mean that the sensor is good.

Have someone test the sensor itself.