I have been a manager at Internet-related companies since 1995. In that time, our companies have always had some form of Internet access available to employees.
As a manager, I have learned there are at least three main ways for colleagues to hide surfing somewhere you shouldn't on company time. The first is the most obvious - obstructing the screen, either with something - work papers, a coffee cup or, if all else fails, themselves. The second most popular way is the distractor, who when confronted with personal things on their screen, deflects all eyes elsewhere. "Hey, is that guy holding a bow and arrow?!? No?"
The best way is the old quick click - minimizing the screen or flipping to another to avoid detection. If executed well, it is almost undetectable. Except you have to move your hand.
I have had more than one employee over the years who every time I walked into their office/cube, they felt the need to quick click.
I am a trusting guy, but if you have to stop doing something personal every time i walk past you, pretty soon I think I will begin to believe you aren't doing much at work.... And I suspect that is a slippery slope.
So, don't quick click every time. Sometimes, just leave the page up as it is. It is less suspicious, than the quick click, I promise! (or you could quit surfing so much at work and just work more!)