The point that only one reply touched on is that you clearly have a
coolant leak.    The overflow tank only allows coolant to go into the
system as it cools down (the coolant is sucked into system from overflow
in response to the vacuum created as hot system cools down - if it isn't
full).  Any time you have a leak, it will manifest it self as your
overflow tank being empty  (or at least lowered) after engine has been
allowed to cool down, typically the next morning.  A small leak around
the heads can easily not actually drip.  In fact, possibly the most
likely is a cracked head.  I have one in my westy and have driven it
15,000 miles, simply topping off coolant in the morning.  When engine
gets really hot, coolant is either drawn into combustion chamber or
seeps out, but in such a fine amount that the source is not readily
apparent.  In mine, I can distinctly smell coolant when this is
happening, but it never actually drips.  As the one post pointed out, a
leak anywhere in the system will manifest itself in this manner.  I
think that only around the heads is it hot enough to evaporate coolant
without allowing an actual drip to form.

Dan Walker
Ann Arbor, MI