Wondering if anyone can help here...engine was overheating due to obvious lack of antifreeze,
but there are no obvious signs of leakage anywhere. Pressure tested the system
at like 20lbs for 30 minutes, and not a drip anywhere. If my main fill tank was down where did
it go?

Here's the situation:
'89 westy overheated and light came on the temp guage a few months back.
Took it into the shop and they checked everything, flushed it, pressure tested the lines, fan works, etc and no obvious
cause. After they do all this though, the guage stays normal and seems fine so drove it away. 
A couple of months later it happens again. Only this time, its clear why: the main antifreeze tank is low, so we've lost coolant and that's the cause. Everything else in the system including radiator fan checks out. Great. So refill and everything seems cool, for now. But the shop can find no obvious signs of leakage anywhere, not even a stain, and there's nothing obvious that says where the lost antifreeze was going. 

Anybody have any clues as to the nature of the problem, given a)thermostat works, b)fan works and comes on at the right time, and c)no obvious signs of leakage.

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