California Wiretapping Law
"California's wiretapping law is a "two-party consent" law. California makes it a crime to record or eavesdrop on any confidential communication, including a private conversation or telephone call, without the consent of all parties to the conversation. See Cal. Penal Code § 632. The statute applies to "confidential communications" -- i.e., conversations in which one of the parties has an objectively reasonable expectation that no one is listening in or overhearing the conversation. See Flanagan v. Flanagan, 41 P.3d 575, 576-77, 578-82 (Cal. 2002). A California appellate court has ruled that this statute applies to the use of hidden video cameras to record conversations as well. See California v. Gibbons, 215 Cal. App. 3d 1204 (Cal Ct. App. 1989).
If you are recording someone without their knowledge in a public or semi-public place like a street or restaurant, the person whom you're recording may or may not have "an objectively reasonable expectation that no one is listening in or overhearing the conversation," and the reasonableness of the expectation would depend on the particular factual circumstances. Therefore, you cannot necessarily assume that you are in the clear simply because you are in a public place.
If you are operating in California, you should always get the consent of all parties before recording any conversation that common sense tells you might be "private" or "confidential." In addition to subjecting you to criminal prosecution, violating the California wiretapping law can expose you to a civil lawsuit for damages by an injured party. See Cal. Penal Code § 637.2."