2000 Chevy Silverado Coolant loss

I have been loosing engine coolant every2-3 months for over a year.I have had a pressure test done and no problems found.There are no signs of any leaks at all,no oil contamination from water,nocracks in hoses,no overheating problems ,ac/&heat work fine.No thermostat problems.But after I am through driving and I park the truck and shut it down,I can smell the coolant on the front drivers side of the truck.I have checked my repair manual and there is a mention of a "Vacuum Atmospheric Valve",but it does not show where it is.I'm just guessing that since I can smell but not see a leak, the word"Atmospheric" would be a place to check.