Mine has to be pretty full to eliminate the light coming on. After replacing the expansion tank (surge tank you call it), it took several weeks for the levels to 'level' out where i wouldn't have to top it off with coolant to keep the light from coming on. Just keep filling it up to eliminate the light (not to the top) and if it keeps going down on ya..you most likely have a leak and will have to start looking for dried orange goo.

 

2003 Expansion tank has a level in there. If you remove cap and look down in there, you should see a little step looking thing. This is the level guide. If your coolant is below that, fill it over that step.
You need to think about the last time you checked/topped it up. If it has been quite a while, it is normal for it to be a little low. Fill it up and drive it. If light doesnt come on you are a-ok. If it does, start looking.
The sensor is on the bottom of the expansion tank. Very hard to see without getting down in there.

 

Ok so here we are 5 years later, got the light to go out for all this time and now it has started again. When I leave home in the morning the light is out, I drive at highways speeds for about 5 km and on comes the light and remains on for about another 10 km and then it goes out for the rest of my drive to work. Outside temperature does not make a difference. Recovery tank is filled right to the top both cold and hot. Any solutions would be welcome