Quite sometime back I went through this invalid traffic scenario on sites which:
a) Had no paid traffic
b) Had traffic which was from links embedded in emails, or elsewhere citing me as an authority on "How to paint Blue Widgets Red"
c) Had traffic from links embedded in web sites as an authority - from similar genre sites, educational and some government sites.
d) Traffic from bookmarks where I was used as a text book
e) Search engine traffic
And as Netmeg said, I was :
content page with a little bit of adsense
I worked with AdSense for a few months on this, they may have learned something, I certainly didn't - because I am still none the wiser.
Then the problem disappeared as fast as it arrived. To be fair, AdSense did reward me modestly for my time involved assisting them with an unexpected financial bonus.
I still don't know what happened except for the fact AdSense engineers monitored traffic closely on a minute by minute basis for quite some time. Obviously they wouldn't tell me anything apart from implying I was a "squeaky clean victim".