I believe the overflow tube exits in front of the right side rear wheel so the problem would show up exactly as you've described. If the coolant tank ruptures there's a good chance you'll find coolant in the trunk compartment as well as on the ground. I've replaced my cap twice with no other leaks present. Try the cheap stuff first.
I just went back to the car and there was no leak under it, and a brief feel under the styrofoam didn't reveal any moisture - hopefully this means you are right!
Check your temp gauge and watch to see if it is getting over 200. You may need a coolant flush or even a water pump. If is ok and no more dumps, it may have been a fluke thing.
Old Hippie Young Heart
2000 S/3.2 Liter/Tiptronic/Boxster S Sport Package/Cruise Control/Slate Grey Metallic
Red Special Leather Interior/Red Floor Mats/Red Hand Painted Instrument Dials/Roll Bar/Windstop
Small Carbon Package/Leather Wrap Carbon Wheel/Center Console Exterior Color/Alum Carbon Shift Knob
AM/FM Radio w/CD Player & Changer/Digital Sound Package/18" Turbo Wheels/Wheel Caps w/Colored Crest