In law school, I was always in the VAST MINORITY on this issue. By FAR, the majority of my classmates seemed to believe that the 4th amendment grants people the right to be free from nearly all interaction with the government of any kind, unless a warrant is issued upon probable cause. The majority opinion seems to be that the police need to avoid making eye contact with anyone in public, lest they inadvertently and improperly "search" the external appearance of a citizen, in violation of the 4th amendment.
I wholeheartedly disagree. I believe that the 4th amendment grants a strict right: The right to be secure in your papers, persons, houses, and effects. In other words, I believe that the police do not have the right to come into your house, disrupt your life, disturb your posessions, and otherwise place your life on hold while they conduct a search of you and/or your stuff. However, I believe that this is where this right ends.
I believe that if you are out in public, in a car, for example, that the police should be able to search your car, so long as they do not unreasonably detain you without cause. (Detaining a "person" would amount to a violation of the 4th.) I also believe that if the police can view your activities from a public place, without you knowing about it, then this is not a violation of the 4th amendment. Sureptitious surveillance does not amount to a disruption of your "persons, houses, papers, and effects." Essentially, you carry on about your life, and you never know the police are watching.
I have "lost" this debate on numerous Constitutional law cases we discussed in law school. The one I remember most had to do with whether the police could use heat-seeking equipment to look into a house and spot a hydroponic marijuana garden. I said that the police should be able to do this, because the heat is viewed from a public place. The courts said otherwise.
Another case had to do with cops flying over a warehouse full of drugs, where there was a hole in the roof and the cops could see the drugs from the airplane using telescopic photography. I said, "It's in plain sight!" The courts said otherwise. Because the cops had to use telescope equipment, and because it was "apparent" that the warehouse owner had "tried to conceal his activities" by putting a "roof" on the warehouse, this was a violation of the 4th amendment!
It seems that the scope of the 4th amendment has been stretched and stretched, until now it seems that the cops must avert their eyes to public acts of violence for fear of illegally "searching" the public streets!