Smoke machines arent as good as they are made out to be. They wont show a leak in a canister purge valve or most small leaks, as the leak is small so may be the output of the smoke, and it can precipitate before it gets close to the leak.
I would pull the purge valve and look for carbon canister debris( carbon media ) in it. It is a known fact that GM canisters have an issue with the carbon getting loose and being sucked up into the purge valve.If you see it in the valve or not, disconnect both ends of the line and blow it out, look for the carbon media when you do.carbon media will require canister replacement after repair/ replacement of the purge valve
Also canister closed valves can get water intrusion into the filter and the dirt can embedd itself in the plastic plunger of the valve and cause it to stick, either intermittently or consistently, not closing setting the code when the diagnostic monitors run.
Look at the filler neck real good first, I have seen many rusty ones and some have holes in them, causing the issue you have.