"what the hell is going on?"
You are low on coolant.
The sensor is in the bottom of the overflow tank. Why you are low, that is a matter of opinion. Could be anything from a leaking coolant hose (and there are alot of them), a leaking radiator, a leaking heater core, a leaking water pump, or a leaking head gasket. It doesn't have to be leaking coolant on the ground, if the head gasket is leaking it will "just disappear" leaving you with an empty coolant overflow bottle.
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