During a midwest winter and maintained highway driving the engine can run very cool due to a thermostat which will not close or is very sluggish. I've had it happen 3-4 times on various cars (after the stat has been in use for several years). The radiator hoses and the block are then barely warm so it is clear that the engine is cold. Also there is very little heat available inside the car. When left to idle without moving if the stat is bad the car will warm up fairly well and the heater will start to be effective. Removing and testing the thermostat in a pan of boiling water proves whether it is unresponsive.

That said, gauges and ECT sensors can of course also go bad.