Unless you're searching some anti-government stuff or instructions for a home made dirty bomb I doubt they'll find anything of interest. I know for sure nothing in my browsing is suspicious (and yes, I don't count internet porn to qualify for that heavens sakes!).
Then again if you ARE prospective terrorist, anti-government or wannabe mass killer you should be very very worried - and so should we.
you make the mistake of thinking that that is all they'll be looking for.
what if you happen to be looking into some form of kink? and what if someone happens to decide that the particular form of kink that you're looking for is bad, and shouldn't be? you've just become a criminal in their eyes.
what if you're looking for somethign as innocent as a harley...........now, in their eyes, suddenly, you're a menace...although in reality, you're not.