https://github.com/RamboRogers/rfhunter
This project is an RF Signal Scanner built using an ESP32, AD8317 RF detector, and various other components. It's designed to detect and measure RF signals in the environment and display the signal strength on an OLED display. It's useful to find hidden cameras, wiretapping devices, and other RF-enabled devices.
(Score: 2) by namefags_are_jerks on Wednesday October 30, @02:26AM
RF Signal detectors are over a century old. That ESP32 would have like 400 million transistors (as a guess); you only need one.
The old-tech Detector diode way would also pick up other RFI, which would be answer your question -- the ~150 kHz RFI from DC power convertors, the 10(*N) MHz harmonics from wired ethernet, and the other birdies from electronic devices. Like you said, a smart creep would cloak the recorder's RFI with another nearby RFI source.
The old 'FM Radio Bugs' kits that're entirely analogue and whose emissions are within the FM band would be quieter.. (although there are of course methods for detecting those)