Who's got the herb?

If you're in the United States and police knock on your door asking to come in to inspect for a suspect on the loose, you should probably hide your drug stash if you have one. There is a "plain view" doctrine that says if evidence of a crime is in sight, police can use it against you in court (even if they weren't setting out to find it).

U.S. Manhunts: What You Need to Know

One of the first things that happens in a manhunt on a more regional scale is to set up containment -- a term that means one or more officers can see any exit the suspect might take. That doesn't always just mean that a couple of police officers eye the block. Vehicle checkpoints or K-9 units might be used to sniff out a perimeter. After that, law enforcement will quickly set up a command post, where officers (or the press) can be briefed. Note: Location can be tricky. If you happen to be reading this and setting up a command post, be sure that your suspect isn't close enough that he or she might hear the briefings. It's happened.

In the case of the 2013 manhunt for the Boston Marathon bombing suspects, the containment went far beyond the usual perimeter security. That was for some fairly significant reasons: First, the suspects were armed with explosives. Second, after one suspect died, the other fled into a residential neighborhood. The police solution in this case was to put up a containment area of 20 blocks, while shutting down public transportation and putting residents in the restricted area on lockdown in their homes. Most manhunt containments are a little more lenient though.

If you're caught in the midst of a lockdown during a U.S. manhunt, there are a few things you should know about your own rights. The law says the police can't just burst into your house looking for a suspect without probable cause. That means that if police see or hear something that gives them reason to believe the suspect is there, they can go in. (They are allowed to go through lawns and peer in windows.) They can also enter without permission if they need to provide emergency services. Of course, they can also go door-to-door (like in Watertown) and look around your house if you give them permission.

But here's the big one. Exigent circumstances (an emergency) might also allow them to enter a private residence -- and, arguably, a terror suspect armed with explosives might be the emergency that would impel officers to act without a warrant. The gamble is that if their intrusion is deemed unlawful later any admissible evidence is thrown out.

As you can imagine, not every manhunt is conducted by the area police department. Depending on what the suspect has done, several different agencies might take over jurisdiction. For instance, we'll talk about a manhunt that took place for a "mountain man" charged with pretty minimal misdemeanors and break-ins. That years-long search was conducted by the local sheriff's department. Oppositely, the Boston Marathon bombings were an all-hands on deck affair, involving everyone from surrounding police departments to the FBI -- thousands of officers from dozens of agencies [source: The Boston Globe]. An international case like bin Laden was led by the CIA but included collaboration among many federal and international agencies. Most agencies are very aware that such communication and collaboration is essential to a search.