There’s two seals on a gas cap. The outside rubber-o-ring you cleaned and vasolined, which forms an air-tight seal where the cap meets the filler tube entrance, and a second one that isn’t visible unless you can take the gas cap apart. The 2nd is a spring-loaded one way valve that allows air into the tank to replace the space for the fuel as it is used up. The incoming air path bypasses the first seal, otherwise it wouldn’t work. But the 2nd seal has to seal the tank from gas fumes going in the other direction too, just like the first seal. And that seal or its seat also can wear out or be defective right out of the box, which could cause an evap leak code.
Since you presumably now have one gas cap you’ve taken out of service, you could try to disassemble it and find the spring-loaded one way valve if you wanted to give it a go, so you can see what I’m talking about. I’ve done that several times myself. It has a good size but quite weak spring that can come in handy for use in other projects.
Leaks are annoying. Plumbing leaks, air leaks, all leaks. When you got one causing you grief, there’s often no simple way to tell where it is leaking; the only thing you know is you got a leak. About all you can do is eliminate the possible sources one by one. Starting with the gas cap makes sense b/c it is easy and inexpensive. If that doesn’t do it, then you’ll have to move on to the other possibilities. What those all are varies car to car. But the canister vent valve and the purge valve are both possibilities. I’m presuming the canister vent valve is designed to be open during tank re-fueling to allow air to be pushed out, but closed when the evap system is being tested by the computer for leaks. There’s often a valve on the top of the tank or near the tank too. And of course any of the hoses or hose connections can leak. If you could somehow get the computer to go into pressure test mode you might could brush soap bubbles on the suspect to find a leak, like you’d find a leak in a tire, but I’ve never heard anyone here say they do that on evap systems.
I’ve never had to worry about this problem myself b/c the evap system on my Corolla is much simpler than the way they are designed these days. The downside with the older designs like mine (for the environment) is there’s no way for the computer to periodically test whether the evap system is leaking or not. It is only tested during emissions testing. What they do is hook up a special gas cap gadget that has a connector on it that allows them to pressurize the gas tank. They must have to clamp off the canister vent path for the test. The test the cap too, with a separate gadget.
Home made smoke machine? I’d be hesitant to use that anywhere near the fuel system unless you can guarantee it is designed in a way that it won’t output anything hot, especially an ember or spark. I think the pro smoke machines use some kind of chemical reaction to make the smoke. Those only output room temperature smoke.