Your cap vanished because of the known design flaw in the cap/reservoir. You'll eventually lose it again and the caps are about $30, with the discount. You are lucky the cap didn't rest on your headers and catch on fire.
Ensure you order a HiPo Engineering - Power Steering Vent Tube with your new cap and prevent another loss or a fire. High quality, inexpensive $20 little part in chrome or nickel.
www.hipoengineering.com