I had this issue. It turned out to be a bad gas cap. Here's a good piece of advice; If you do buy a new gas cap, don't cheap out. I replaced my original gas cap because the tether was broken.
I bought a $15.00 one from amazon and after I put gas in my car the gas light started blinking. I disconnected the battery and it stopped for a day, then it started blinking and the check engine light came on.
I had the codes for large evap leak and small evap leak.
Then I bought the correct cap for $33.00, for an SLK 320 and it's been fine ever since.

I don't know if this is your problem or not, I'm just saying what mine was.