Receptional - 10:06 am on Feb 21, 2013 (gmt 0)

Here are some ideas. They may be wrong...

Although the site is not indexed, there may be ways in which Google sends you traffic (or appears to send you traffic). If users move FROM a Google page to your site, for example, some tracking systems may call that traffic from Google. Aternatively, images or frames might be embedded on third party sites. Thirdly - the Google traffic MIGHT be Googlebot itself crawling the site (again depending on the tracking system). But my GUESS is that your site keeps on getting indexed then de-indexed through a filter - so you occasionally get some real traffic from Google - but as soon as it comes in, it gets de-indexed again.