2002 Beetle loss of coolant
 

I have a 2002 VW Beetle that after driving a few days, the coolant level drops and the red temperature icon comes on. There does not seem to be an visible fluid leaks under the car and after adding more coolant, the light goes off, then after a few more days of driving, the same thing happens. There seems to be no overheating problem or boil over. This is confusing so if anyone can offer some advise, I would appreciate it.
Thank you

I also have noticed that only one fan turns on. Are both supposed to run at the same time? Still no visual leaks from under car.
Thanks for any advice or recomendations