Why would my low coolant light be on if I have a full reservoir of coolant?

My 2002 Ford Escape's low coolant light is on but there's no leak of coolant and coolant is full. Turned on last night and I filled it up with coolant because it was low. The light come off. I thought there was a coolant leak at the bottom because I noticed a puddle of liquid but instead it's clear and odorless so it looks like there's water underneath leaking. I did notice a tiny bit of coolant on the reservoir plastic container on top that I thought I wiped away before. It's not overheating and the AC runs very cold. It's hot out today so I thought I'd have overheating problems. Any ideas?

Added (1). Added note, this car does not have a radiator cap to fill at the radiator. You only fill at the reservoir. I looked at the hoses leading to and from the radiator to the reservoir and I didn't see any leaks or cracks.

Check the coolant sensor and wires and the coolant cap

There's a coolant level warning switch on the radiator that may have gone south on you. It's usually a $15 part and easy to fix on most cars. There's also the possibility of a coolant bubble but if it wasn't leaking I would go for the switch. I will check for you…

OK, the level switch is an integrated part of the coolant reservoir - http://www.partsgeek.com/gbproducts/DC/5217-05054215.html?fp=pp&gbm=a

Check the recovery reservoir tank for a wire connector on the bottom of the tank.

BTW; this will probably be 2 brass buttons full of corrosion in the tank and you should be able to find them by the black electrical connector in the pic. They may be corroded together and shorted out.

The sensor is probably bad. I had that with my daughter's 2004 Chevy Classic and it turns out only a new sensor would turn off the light - disconnecting or shorting the sensor did not good. Yours may be different.