"It is one thing for a passerby to observe or even to follow someone during a single journey as he goes to the market or returns home from work. It is another thing entirely for that stranger to pick up the scent again the next day and the day after that, week in and week out, dogging his prey until he has identified all the places, people, amusements, and chores that make up that person's hitherto private routine."
Specifically, the court found that the whole of the activity may add up to more than just the sum of the parts. While this makes sense, there are a lot of questions as to whether it goes way beyond what the law allows. Eugene Volokh Orin Kerr notes a key problem:
Much of the problem is knowing when the line is crossed when a bunch of non-searches become a search. The Supreme Court has stressed the need for clear rules that tell the police what they can and cannot do. But how do the police know when a mosaic has been created such that the sum of law enforcement techniques, when aggregated, amount to a search? Are they supposed to carry around a D.H. Ginsburg Aggregatormeter that tells them when it's time to get a warrant? Take the case of Maynard. One-month of surveillance is too long, the court says. But how about 2 weeks? 1 week? 1 day? 1 hour? I have no idea.
Even stranger, the mosaic theory has the bizarre consequence of creating retroactive unconstitutionality. The Maynard opinion indicates that it would have been okay to monitor Jones for a short time. Let's say that would allow monitoring for a few trips over the course of one day. At the end of that one day, the first day of monitoring would be constitutional. If the prosecution wanted to admit that evidence, it would be fine. But by continuing to monitor the GPS device for more time, that first day of monitoring eventually and retroactively becomes unconstitutional. It becomes part of the mosaic, and the key point of Maynard is that the entire mosaic is considered one entity.
I have to admit, I see both sides of this argument. The full on, warrantless GPS tracking certainly does seem to go beyond what's reasonable. But the vague standard could create some problems as well (even if I generally like the idea of looking at the whole of the activity, rather than trying to focus in on the individual pieces). It seems like the good way out of this would be for Congress to protect privacy by requiring a warrant for such tracking, but there's little chance of that happening. Instead, what's likely to happen is that the Supreme Court will be forced to weigh in on this eventually, and my (very premature) guess is that they'll side with law enforcement.