One site with low traffic isn't going to make a difference. I have five sites that the one you described. The ctr runs around 15-20% but the sites have very low traffic. I believe that Google looks at the entire account and total averages when looking for fraud. Another thing that they probably look at is the monthly average for the site. Lets say the site with the high ctr earns $20 per month on average. Should you start earning $20 a day that could throw up a red flag.