How do you know that info in the first paragraph?

On top of that security, it'd also be interesting to monitor what someone (based on their IP and browser fingerprint) was looking up on SO, you could probably figure out what they were working on that way.

Although I guess when you work for the spooks, they could figure out how to mask your IP that each query would come from a different IP from the world (basically I'm describing Tor, I wonder if they have their own Tor network).