[Vwdiesel] Coolant leak

[Doug]

I have a coolant leak that seems intermittent but will lower the 
reservoir level over a few days and leave a few spots on the garage 
floor. I have checked all the hoses and the water pump and cannot find 
any traces of antifreeze on them.I thought I had found the leak when I 
found coolant on the heater control valve. I changed the valve and the 
leak seemed to be cured but reappeared after a few days.  Is this type 
of leak indicative of a head gasket issue? I would expect to see black 
spots in the reservoir if my head gasket as leaking. Can I have a head 
gasket leak to the outside that doesn't show up in the coolant 
reservoir? If the head gasket can leak like this, are there any 
preferred locations where the leak is most likely to occur? The motor is 
a turbo motor with turbocharger installed in a 81 Caddy.