gas cap would have been loose or the gasket starting to crack. vent valve is a little harder to check, you have to take it out, disassemble the filter and clean with air. then actuate it with some scrap wires and a battery, you should be able to blow through it then not and the solenoid in there should go back and forth smooth. shoot some wd40 in it actuate it a few times and put it back. if it doesnt move good you need a new one. the one on my old 03 monte which i think would be similar location was tucked up in the fender behind the wheel.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock