Reason I ask is that one of my sites gets around 1,000 visitors per month via organic search in Google. A good number of them view on average 3 to 4 pages and stick around for an average of 3 minutes. So in my view they are interested in the info on the site. So do they leave a footprint of their details just by visiting and could those details be harvested in some way. If so I could then possibly contact them politely to enquire if you require further info.
I get the feeling I might be going down a Black Hat trail with this. Is there a White Hat method?