The Fourth Amendment of the U.S. Constitution protects against "unreasonable searches and seizures" – but what does that mean when it comes to techniques that use technology rather than a physical search that is easy to see?

In many ways, it remains unclear. But there a few key issues that courts have been considering lately when it comes to this question.

First up: whether the activity being observed by the technology is outside or inside a person's house. Courts have consistently ruled, for example, that the use of a GPS device to track people outside their home is not a "search" under the Fourth Amendment. The idea here is that if people are doing things in public, they don't have much expectation of privacy. A similar argument could extend to the use of cellular signals to track people via their phones outside their home.

Inside a home, though, things are different. The first part of the Fourth Amendment gives people the right "to be secure in their persons, houses, papers and effects."

In a case in 2001, the Supreme Court said that the use of a thermal imaging device that detected large amounts of heat radiating from a garage constituted a Fourth Amendment search. This was despite the fact that the device was used only outside the house and detected only heat that was emanating through the walls.

The issue of technological searches inside homes is a question being raised in the case of alleged "hacker" Daniel David Rigmaiden, which is currently in U.S. District Court for the District of Arizona. In that case, which was the subject of a Wall Street Journal article today, federal investigators used a device called a "stingray" to locate a mobile broadband card inside an apartment building.

Stingrays and similar devices mimic cellular towers and can force a connection with the phones or broadband cards they are tracking. The government in this case said the stingray actively pinged the mobile broadband card while the card was in an apartment and got it to respond to the stingray so that investigators could locate the card.

Whether investigators got a proper search warrant in that case remains the subject of debate.

In addition to the location of the search, courts also have been looking at how invasive a technological search has to be before it requires a warrant.

This fall the Supreme Court will hear the case of Antoine Jones, whose truck was tracked by police for a month before they got warrants to search for drugs in places he had visited. One argument here is that, even though Jones was traveling in public, the amount of time he was tracked amounts to an invasive search and is subject to Fourth Amendment rules.

Invasiveness of technological searches has been an issue for a while; a 1979 Supreme Court case found that investigators didn't have to have a warrant to get phone numbers a person had dialed. This is different from a wiretap that gets the content of a conversation; that's invasive enough to require a warrant.

That 1979 case also brings up something called the "third party doctrine," the idea that if a person knowingly gives data to a third party, they lose their Fourth Amendment protections for that information.

The prosecution in the Rigmaiden case, for example, has argued that Rigmaiden does not have an expectation of privacy in part because he knew the broadband card would have to communicate with his cellular provider and disclose information about its location.

With Internet data storage and increasing amounts of information being sent to third parties, this is rapidly becoming a larger question for courts and lawmakers to handle.

"You cannot communicate these days without using a third party," said Christopher Slobogin, the director of the Criminal Justice Program at Vanderbilt Law School. "That would mean the government never needs a warrant to obtain information about communications or location."

Related article: 'Stingray' Phone Tracker Fuels Constitutional Clash