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My car is a stock 96 Z26, 140 K miles, 3.1 V-6 engine.  The intake gasket, water pump, and radiator have all been replaced in the last 18 months.

Here's what happened:

For a while now, I've been thinking that I smelled antifreeze while driving.  Since no one else could smell it, I thought I was paranoid after all the other issues I've dealt with in that department.  So, the other night my car overheated.  If it makes any difference, I drove for a good 8-9 hours total that day.  When we popped the hood and looked around, we found that there was pretty much no antifreeze left at all.  We put in the antifreeze and water I had with me, let it cool down again, and drove home with no problems.  When my dad checked it out the next morning, he couldn't find a leak anywhere.  This was after letting the car warm up for 20 minutes on its own, then driving for another 15.  The overheating happened Thursday night, and I haven't had any problems since.  

Here's my question:

Any idea what could possibly be going on?  What signs I should be looking for?  Any help to prevent further damage would be greatly appreciated.  Thanks!

Do me a quick check.  Go out and check your oil.  Then report back.

Level is fine, but it is time for a change.

Is it milky?
Check to make sure the coolant is not burning off the side of the engine...  It could be a lower intake gasket leaking outward instead of into the internals.
Also check to make sure the heater core is upto par.

To properly check all this, go rent/find a Pressure check for the rad cap and hook it up giving it a 15PSI pump.  Let it stand for a few minutes, but watch it still during those few minutes.  See if it drops down in PSI.  If it does, then you have a coolant leak somewhere for sure.

Thanks, I'll let my dad know and see if that helps him at all.

So I assume the oil was not looking like a milky substance?

Nope, that was the first thing my dad and I checked for... that happened the first time I couldn't find my antifreeze; it all went into the engine block.  That was fun....