Coolant switch problems?

["Jeff Loh" <jeff.loh@fluordaniel.com>]

     Audi gurus:
     
     I mistakenly loosened the coolant expansion tank lid while the 
     engine was still warm, and lost about 1/2 cup of coolant.
     
     Now the coolant light keeps blinking at me. 
     
     Is it _that_ sensitive to coolant level? Which makes me wonder. 
     Is the switch level actuated or pressure actuated? Since the 
     coolant was overflowing the expansion tank I'd assume it wasn't 
     level actuated. When the engine is off (and cool) is the coolant

     system still pressurized?
     
     Just curious as to why.
     
     /J.
     80Q