Well the hot and cold lines on the coolant res. are for levels when the car is dead cold, then fully hot.
You may have a leaking radiator. I had this issue and it turned out there was a small crack right at the neck where the upper hose attaches that would open up when hot then close back up as it bled off pressure. The only reason I found it is I happened to notice a small trail of dried up Dexcool running from it. Since the side tanks and necks are plastic on these cars its a good place to start. Not much over there besides the hose and thermostat. The only other place I've had leak is between the upper intake manifold and the lower right around the TB, but that was more of a steady drip and didn't cause any steam to form.
If you can't catch it leaking from somewhere I may suggest getting a pressure test done on the cooling system. It should show any cracks/leaks that way.
