I'm no expert either, but Horatius makes some good sense there.
Part of the problem with the coolant system is that to detect moisture levels usually requires some form of current to be passed through the liquid, detecting it. Modern coolants have anti-conductor additives to reduce damage to the cooling system (both the water jacket of the motor and the radiator) through corrosion - because corrosion is caused by oxidisation, which required the flow of electrons.
That doesn't mean it's impossible. The ultrasonic sensor might have some merit if you could rely on the system to always draw from the expansion tank first. If there's a coolant leak, though, that may not happen.
What would be more reliable is some means of measuring the height of water in the radiator itself. Imagine adding a 90 degree threaded elbow to the top and bottom tanks of the radiator and putting a clear hose between them - you have a sight glass. Use the same technique to put a float level inside - you solve the problem with non-conductive fluid and detection of level dropping.
Can that be done reliably? That's the key question. Has any manufacturer developed a method of determining whether or not the coolant level in the radiator is dropping?