Can law enforcement record a conversation without informing?

Answer 1

It depends on what state they are in. Some states require all parties to a conversation to be informed that it is being recorded, though even in those cases the law may contain an exception wherein law enforcement is still allowed to record without per se informing the person they are talking to. For example, most police cruisers are equipped with dash-mounted cameras that are linked to a microphone on the officer's uniform. While officers sometimes will inform a person during a traffic stop that they are being recorded, they don't necessarily have to in most jurisdictions.

Recording a conversation with an obviously uniformed officer is distinct from secretly recording a conversation where the person does not know law enforcement is involved. In these cases, a warrant or other authorization is almost always required to do so. Failure to do this can result in all evidence obtained this way being inadmissible, and even criminal prosecution for the officer in cases of extreme misuse.

Added: Except for certain enumerated matters concerning 'national security' law enforcement and the prosecution had better have a warrant or the results of their effort will be for naught. Any and ALL information gained from the unlawful recordings will all be thrown out under the "fruit of the poisoned tree" doctrine.