Taking it a step further, what about tracking your local concerned citizen who is a bit vocal about the issues they care about, attending protests and the like? The FBI has been known to use GPS-enabled cell phones to track political activists even if they're peaceful and nonviolent. While the government hardly has the best track record regarding even mild dissidents, this has led to a considerable amount of alarm amongst human right organizations including, ironically enough, the very people the FBI are tracking.
There are all sorts of shades of gray involved with who's to be tracked and who's to be left alone. Pedophiles who committed their crimes decades ago are often tracked, just as other criminals who really may have moved on with their lives.
While it certainly can be argued that such tracking of individuals may result in a few captured criminals here and there, many also wonder whether this is worth the loss of human liberties. It's a question that's up for debate.