Twice in the last 3 weeks the low Coolant warning came on, both times I let the car cool overnight and added coolant the next morning and the reservoir only took about 1/2 liter if even that much before reaching the top.

Is my coolant float sensor defective or is it supposed to be so neurotic about low Coolant? Or is this symbolic of another issue?

Temp gauge is always normal, never goes past the half mark.

 

They can be a bit touchy....keep an eye on it. ;o)
I have found that my coolant level stabilizes if I leave it about 6mm/1/4"
below the filler neck and then it never moves.
What sort of mileage has the engine done?
Check the operation of your level sensor by giving it a poke with a slender stick or biro to make sure it's moving up and down freely.
Check the wiring connector below the expansion tank.

 

twice 1/2 liter is IMO not little, and indicates something is wrong.

 

Yes, that's too much. Try replacing the cap for the pressure reservoir first. It's inexpensive and if it does not work right the fluid sent to the overflow tank will not be drawn back when the engine cools.

 

I would be very worried about a hose leak you cannot see, probably in the V valley, or thermostat housing.
Should not need to add coolant at any time.

 

Your points were taken to heart, thank you all, I examined the coolant lines and found one of the hose clamps had shifted about 5mm and was letting coolant out from the mating point between the hose and the tube it attaches to. Easy fix.

 

Be interesting to know which tube/hose was involved? ;o)

 

The big one at the top front of the engine block right in front of the fan, connecting to the thermostat tube.