Question, what controls the "check coolant level" warning light..?

I replaced the thermostat, waterpump, belts etc on my '04 X 3.0, 3 weeks ago and everthing has been working perfectly since.
Over the weekend the check coolant level light came on after a routine fluid level check and it won't go off. The coolant level is perfect cold and hot and I get a good flow via the weep hole.

I had to disconnect a sensor on the thermostat and one on the lower radiator hose when i did the job but they are plugged in and have been checked. I don't know if the sensor in the new thermostat controls this light in which case i may have a bad new stat??

Help and suggestions appreciated.......