89 100 Radiator getting low fast?

[Martin W. N. Suryadarma]

Hi all,

In the past month or so, ever since it got cold, the radiator reservoir
has been getting low very quick. I would drive it from home, then the
warning light will come on. So I had to pull over, looked in the tank, and
it's 1/2 empty. So I topped it with the coolant. Then driving home on
the way the same thing would happen again. I don't understand how this is
possible. I don't see any leaks or anything. I mean this looks like big 
time leak, but I don't see anything under the car when it's parked. Any
hints/helps is greatly appreciated. The car is 1989 Audi 100. Thanks.

Martin