Olmstead v. US was about particular places (excellent dissent by Brandeis, harkening back to his original privacy article). That's what got overruled in Katz: the idea that without a physical trespass there was no 4A violation. But there is discussion (in the dissent) that a telephone call is like a sealed letter, and that wiretaps are an invasion of privacy.
Now heading inot Katz, we get the idea that when you expose information to one person, you risk exposure to everyone. Sort of a waiver-type argument: that's why pen registers are allowed. Katz is a watershed case; you can listen to the arguments at oyez.org, and it's kind of comical how primitive the bugging was. But anyway: 4A protects people, not places. Back on the subject of pen registers, though, we allow the collection of identifying information, but not the content of the communication. Is that a good idea? Note that the standard that gets adopted is actually from Harlan's concurrence:
So what determines whether somebody acts as though they have a reasonable expectation of privacy? What if Katz was just talking super-loud in the phone booth? Or what if he left the door open? Does the location of the microphone matter: if it had been in the bushes, and not on the phone booth itself? The test in Katz is dofficult to apply, and we haven't even talked about the use of sense-enhancing technology yet. But after all, why should it be your expectations that are protected? Expectations are conditioned by circumstances-- why shouldn't we talk about what privacy people actually should have?
On the other hand, the justifiable reasonable expectation of privacy is a nice and adaptable standard. And then we get Miller: pretty much all people on the street might think this case came out wrong. Just because I share my financial records with my bank, they can be collected? But it seems so, on the same logic as the pen registers.
Think about the purpose of the bag in Greenwood, though. There we have much more than just finding out that trash was being thrown away: it was the contents of the bag. But at the same time, the bag wasn't there to conceal garbage, it was there to contain it. Note that there are varying degrees of protection (e.g., for dumpsters, vs bags, vs containers supplied by trash picker-uppers). What is the "reasonable expectation" with respect to your trash?
Interesting point: after Katz we started getting phonebooths without doors on them.
And we get the distinction between Knotts and Karo: that's just based on presence in the home.