Mystery coolant leak

[BriceW at webtv.net]

I took a short drive yesterday and my low coolant light came on. I
pulled into the garage and noticed a big puddle of coolant on the floor.
I opened the hood and sure enough the expansion tank was almost empty.
I then re-filled it and started the engine up to find the leak. No leak
appeared anywhere. I then test drove it and went over 60 MPH with the
air conditioner on lo cool and high fan. The temp outside was 96.
Still no leak.
I have checked every hose and nothing.
A month ago I did the neck repair on my half plastic original radiator.
I checked the repair and it was holding fine.
Could the plastic on another part of the radiator have a hairline crack
that only leaks sometimes?
I have been waiting to replace the radiator becuase I want to get the
Nissens #60437 all metal one.
I am very puzzled by this and am not sure where to look.  Now that I
want it to leak it wont leak!
Now I am afraid to drive a long distance especially without carrying
gallons of coolant with me.
Also should I try one of the radiator stop leaks products.
I have 102,000 miles.