I am having the same problem. My mechanic doesn't have a smoke machine (small mom and pop shop) but he checked the entire system looking for bad hoses/leaks/etc and never found anything. He did replace the gas cap but we're waiting on a new OEM fuel line to be shipped in (unrelated issue) so don't know if the cap has solved the problem yet. Thankfully since he doesn't feel he fixed it he's not charging me for all the time spent looking through the system.
I think I'm going to have to go to a shop that has a smoke machine.
If you find out what the problem is please post