Could it be that the cap is old and its leaking through there as the coolant is still green and it is leaking from the cap

i.imgu(dot)com/1lQzrKX.jpg

I came home from work and noticed white smoke/steam coming from under the hood, so I popped the hood and the smoke/steam was coming from the intake side. Once I turned the engine off I could hear boiling and noticed all the coolant had boiled into the reservoir tank. But the temp gauge was fine. Not...