440 is the general evap code, could be alot of things, like i mentioned earlier if you had the vac tool you could hook it to the lines and service ports see if they hold vacuum. there are a few different conditions that have to be met before it even checks the evap, its not likely that it would check it in random 15 min drive.
another thing you could check, my monte was getting a 0440 and i tracked it down to a sticky vent valve, i bought a new one from the stealership and it lasted about a year before it was doing it again. i took it apart and sprayed wd40 in there and actuated it a couple times with a battery on the bench and its been 2 or 3 yrs now without issue.
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