In the often cited video showing the law professor and a detective talking about why you shouldn’t talk to police, the detective said that he sometimes uses a small cassette recorder during interviews basically as a prop to trick a suspect into telling more information than he’d do otherwise.
In the interrogation room he’d begin the interview by first starting up the recorder and placing it in the middle of the table so the suspect could see the tape turning and recording. When he got close to extracting the info he needed, he’d then pick up the recorder and stop it, and ask the suspect the important question “off the record”, with the intent to deceive the suspect into thinking it was safe to answer because that particular response wasn’t be being recorded.
After the interview he’d then toss the tape into a box full of other tapes, and either erase it or tape over it during the next interview. The recording made on that device didn’t matter because the entire interview was recorded by the audio/visual recording equipment already installed in the room, including the part the suspect believed to be “off the record”.
So, not undercover police nor a reel to reel device, but there may still be some use for these old recording devices in modern times.