We have really detailed web analytics and the visitor patterns were just so unlike what we normally see. Especially the fact that the same visitor (almost always from an iPad) would come back multiple times with about 10 hours between visits. I am guessing that Google doesn't show the same ad to a user for some time after there has been a click which explains the 10 hours. But we were mystified why people who never left the landing page would click an ad again and again. Once we realized it was kids playing games it became obvious. Many times I have had to rescue my kids who accidentally clicked on an ad while playing an iPad game and gotten lost in some website.