You are right, it should only come on when hot.
By its nature a viscous fan only works when its hot and not when its cold. It will always spin with the engine. When cold the fan viscous clutch slips and the fan does not rotate quickly. When the engine gets hot the viscous clutch does not slip thus the fan blades spin faster. You can hear the fan kick in with a growl/roar as you increase the revs above idle.
With the car stone cold and the engine off, spin the blades of the viscous fan. These should be stiff to push and uniform in resistance. If it spins easily then you have most likely have lost your fluid and it needs replacing.
An electric fan may kick in when the engine is cold if it has a faulty control unit. A viscous fan can never kick in when the engine is running.
But the important thing is that your car is not overheating. The minute that the temp gauge goes beyond normal switch off.
My E39 had a viscous fan and in the three years I had it I only heard in kick in three times. Newer cars don't have them as they are seen as heavy and energy sapping in todays tree hugging emission dictating days of motoring.....
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