I have read a couple of different threads on this cool site - but none of them appear to describe exactly the same problem that I am encountering.
In addition to the Low Coolant light coming on - I am losing coolant. I have seen NOT see the temperature guage climb much above normal, if any at all. I cannot see any visual evidence of coolant loss (e.g. puddles, drips), but I am continually having to fill the reservoir tank every few days.
I have put on a new reservoir cap. I have filled the reservoir and squeezed the LH radiator hose (and yes I was able to add quite a bit more fluid), but it still disappears. And now that the cooler season is beginning, I have noticed that the heater is not getting as warm as it should.
In advance, I thank everyone for any input that you can provide me.

 

I had a loss of coolant and could not find any signs but when I had the dealer look at it they did a pressure test and found a small fracture on the plastic header tank, I think the small leakage was evaporating before it was visible to me.
They fitted a new tank and problem cured, might be worth a look for you.

 

You will get a better response on the X308 forum (1997-2003) as your car is a '98. There have been lots of posts on coolant leaks there.