On the NC700 (I'm assuming the 750 also) the fan is controlled by the engine ECU, using the coolant temperature sensor which is in the thermostat housing, right side of the cylinder head top, forward of your right knee as on the bike. There isn't a temperature "switch" as often found on other bikes, usually in the radiator header tank. The ECU decides when the fan is required and sends a signal to the fan relay, which powers up the fan.
I'd start at the coolant temperature sensor in the thermostat housing, remove the connector, dry/lube (WD40 or contact cleaner etc).
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