One thing is a supeana for your search history after you've been identified and is under suspicion. Then it's a finite number of suspects data. It's another for Google to actively alert law enforcement based on queries coming in.

Now monitoring who buys anthrax spores seems to strike a more reasonable balance between prevention and tyranny.

Think of it this way; going for an evening walk every night isn't (shouldn't) be suspicious. Until you're being investigated for a child missing from the house at the apex of your regular path. Then your walk might have been "scouting the crime scene".