Here's a quick summary of the warrantless search conditions in general (not dealing with the specifics of cars, houses, search incident-to-arrest, consent, warrants, "special needs", or exigency):
To simplify things greatly, the bottom line is that the more intrusive the search or detention, the more justification the cops need.
Baseline: Assume a simple face-to-face police encounter on the street, with the person giving no consent to search, and with no cars, no warrants, and no exigency (emergencies involving great danger) involved:
1) In order to briefly detain you and ask you questions (which you don't necessarily have to answer), they only need reasonable suspicion to believe that criminal activity may be afoot. This is much less than probable cause, and less than a "preponderance of evidence" (preponderance means greater-than-50% certainty in legal-speak), but it must be something more than a hunch. Basically, the cop must be able to articulate reasons for his suspicion, but he can take the "totality of the circumstances" (the whole picture) into account. Terry v. Ohio, 392 U.S. 1 (1968 ) is the seminal case here.
2) In order to be able to pat you down, the cop must have reasonable suspicion to believe you may be armed or dangerous. It's supposed to be a quick check for weapons (meaning they can't go into your pockets), but if the cop feels something that feels like contraband, this may give them probable cause to go into your pockets.
3) In order to arrest you or conduct a full blown search, the cops need probable cause. Probable cause has been defined as the amount of facts and circumstances within the officer's knowledge that would warrant a reasonable person to conclude that the particular individual has committed a crime (in the case of an arrest) or that specific items related to criminal activity will be found at the particular place (in the case of a search). It's more than a hunch or suspicion, but less than proof beyond a reasonable doubt.
The rules get a lot more complicated when dealing with cars, houses, arrests, and the execution of search/arrest warrants.
Anytime you analyze a factual scenario, take the sequence of events, step by step, and analyze them sequentially (as they occurred in time). Start with the first thing the cops do, e.g. a brief detention. Did they have reasonable suspicion or probable cause? If not, then usually, everything that happens afterwards is illegal and evidence that is acquired should (technically) be excluded.
However, if they have reasonable suspicion to do the brief stop, then look at the next thing they do, and ask whether it is justified at the point. What happens during the brief stop may (or may not) give them probable cause to conduct a more extensive search.
So you have to look at the whole chain of events, and at each stage of the interaction, ask whether the cops have reasonable suspicion or probable cause to be doing whatever they're doing at that point.
SUMMARY
1) If no reasonable suspicion -> No stop permitted
2) If reasonable suspicion -> Brief stop permitted
3) If reasonable suspicion that you're armed or dangerous -> Pat-down permitted
4) If probable cause -> Arrest or full-blown search permitted
N.B. These are not predictions about how the police actually WILL behave. They are technical statements about whether the police action in question is legal. As we all know, cops do illegal things all the time. The judge may or may not pay attention to these rules when considering whether to exclude evidence or dismiss charges. Moreover, evidence obtained illegally may still be legally admitted into the courtroom if it would otherwise have been discovered through legal means, or in some cases, if the police make a mistake while acting in "good faith".
[ 02 October 2002: Message edited by: Mahan Atma ]
