Hi Scotty:  2014 Kia Forte 5 - automatic -  1.6 Turbo GDI engine - about 170,000km (approx 105,000mi) on it - and yes, I'm in Canada.  If I let the fuel tank get down below half, then after a few drive cycles I get the check engine light for a P0455 Evap leak.  I haven't had a smoke test, and the only repair attempt was that I changed the gas cap - twice.  The odd thing is that a week or two after I refuel, the check engine light goes off, and my scan tool shows no codes. 

It seems obvious that something about the low fuel level is allowing an air leak, but searching online and reading about how the evap system works I haven't been able to figure out what it could be. As far as I understand it, there's no change in the test procedure, and unless there's a pinhole leak in the gas tank, nothing about the system should change with the fuel level.  And there's never any smell of gas, nor any evidence of fluid leaking on the garage floor, so I'm fairly sure the gas tank isn't leaking!

This has been going on for close to a year now, and I'm not too worried about it (easy enough to run on the top half of the gas tank), but I'd like to understand what's going on.  If nothing else, to be ready for when whatever the problem is becomes permanent.

Any idea what could be causing this?

Thanks