Originally Posted by DBS.US

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."