2004 Saturn Vue Coolant Level light on; not coolant temp li

In the last 200 miles, coolant level light comes on. After warmup now it stays on. Coolant level is perfect in reservoir. Temp gauge stays where it always has been just a little above quarter mark. When we had a dealer they referred to this as the 'Honda engine.' Coolant has not been flushed. I thought this might be a sensor so disconnected connector at bottom forward of the reservoir tank. Engine runs for about 90 seconds before the dash light comes on and stays on. Engine runs fine and 60 mile trips are no problem. Local GM dealer disavows any parts available. I'm reluctant to replace thermostat as temp gauge reads normal. As this problem was developing the light would come on and then go out once car was on the open road and not in slow traffic. Now it comes on and stays when engine at normal speed.