The reservoir sensor is only there to turn on the yellow radiator warning light in the dash. It's not hooked up to the fans. The fans cool the radiator, not the engine. If you're low on coolant, no amount of air will cool the engine.

Under some conditions the clutch fan will come on full tilt during a cold start. IIRC after a few seconds it should disengage and then only come on when it's 105*C or so at the radiator. Maybe that's what you're hearing.

Sixto
... stay tuned