Hopefully you know by now that police need a search warrant to conduct most searches. There are however several exceptions to the warrant requirement, and some police activity that doesn't constitute a search.
Most warrantless searches fall under one of a few main exceptions: if contraband is in plain view, searching a person after he or she has been lawfully arrested, if there is an emergency or an officer is in hot pursuit, and at some checkpoints like airports or international borders.
Here's a roundup of other scenarios where officers don't need a search warrant:
On the Road
Most dog sniffs aren't considered searches in the first place, so they're OK. Unless the officer has to send for the K-9 and it takes a while to arrive. And if it's the cops that smell marijuana, they can probably search your car.
Additionally, officers can search your car if they have probable cause to believe there's evidence of a crime in your vehicle or the officer reasonably believes a search is necessary for his or her own protection.
In the Home
The big exception is consent -- you can't say yes to a search, and then claim officers lacked a warrant, even if your roommate consented to the search over your objection.
In most states, police can search your home if you're on parole.
On Your Phone?
The feds may be tracking data and content on your phones under the Patriot Act -- no one is totally sure. But cops can't search your cell phone without a warrant, even if you're arrested first, unless one of the emergency exceptions applies.
If you think officers have conducted an illegal warrantless search, you should contact an experienced criminal law attorney as soon as possible.