While my story can't quite match yours, it's interesting all the same.
Took the Westy to a local QuikLube place where they check all the fluids,
tires, etc.  I, like you, watch the process to be sure it is done correctly,
but sometimes even that doesn't help.

    I didn't notice till I got home and looked at their check list of
items....let's see, proper grade of oil, windshield washer fluid, engine
coolant, brake fluid, etc.  Only a few problems here...the drivers door was
never opened by them which makes it hard to check brake fluid or window
washer fluid, and as for engine coolant...i certainly hope there is enough
AIR around!!!!

    The moral of thiese stories; keep your eyes on anyone working on your
Vanagon.

Mark
82 Westy
Never lacking coolant, aircooled forever (or at least till the next engine
change)