What are we actually testing?
Low-cost multi-function “bug detectors” are widely sold online with RF, camera-lens and magnetic detection functions. The useful engineering question is not whether the product can ever beep. It is whether the indication lets a non-specialist make a reliable conclusion about surveillance.
Illustrative consumer detector. Features and performance vary widely by model.
This is an engineering guide to the limitations of this class of detector, not a certification or condemnation of one specific model. Exact sensitivity, frequency coverage and magnetic/optical performance vary between products.
A sensible bench test separates detection from identification
A controlled evaluation can present known signals or known objects to the detector and observe its response. The key is then to ask what the alarm actually proves. A broadband detector may be working exactly as designed while still giving the user too little information to identify a surveillance device.
| Test condition | Typical low-cost detector behaviour | What the result proves |
|---|---|---|
| Strong continuous RF source close to detector | Often gives a clear alarm or rising level | RF energy is present nearby; not what the transmitter is |
| Mobile phone transmitting data or making a call | Often alarms strongly at close range | A legitimate cellular transmitter can look “suspicious” to the detector |
| Wi-Fi router / access point | May alarm depending on distance and sensitivity | Normal network traffic is enough to create a positive indication |
| Bluetooth / short burst activity | May flicker, alarm intermittently or be missed | Short duty cycle and low power make simple alarm interpretation difficult |
| Frequency-agile / hopping transmission | May respond to some bursts but not consistently | The detector normally cannot show the hopping sequence or identify the signal |
| Local voice recorder with no radio | No useful RF indication | An RF detector cannot detect a radio signal that does not exist |
| Powered-off electronic device | No RF alarm simply because electronics are present | Other inspection methods are required |
| Strong magnet, speaker or motor near magnetic sensor | Magnetic mode may alarm | A magnetic field is present; not that a GPS tracker has been found |
False positives: the detector may be right and the conclusion wrong
If a detector alarms beside a Wi-Fi router, phone or Bluetooth device, it has not necessarily failed. It may be detecting real electromagnetic energy. The false positive occurs when the user interprets that generic response as evidence of a hidden bug.
Turning sensitivity higher often increases the number of alarms without increasing the instrument's ability to identify their sources.
False negatives are more serious
The absence of an alarm can create false reassurance. A local recorder, store-and-forward device, switched-off transmitter, hardwired microphone or very low-power intermittent signal may not create a useful response. A detector also has finite frequency coverage and dynamic range.
Why the magnetic mode is especially easy to misunderstand
A magnetic sensor can be useful for locating a strong magnet, but a magnet is not the same thing as a tracker. Vehicle loudspeakers, motors, catches, mounts and many ordinary components can influence the reading. Meanwhile a tracker may be taped, screwed, cable-tied, concealed behind trim or hardwired and have no external mounting magnet at all.
What a professional bench can add
Known RF sources, attenuators, signal generators, spectrum analysis and controlled distances allow an engineer to explore sensitivity, selectivity, response time and blind spots. More importantly, the test demonstrates the difference between detecting energy and identifying a surveillance threat.
A £30 detector can be useful as an indicator. It should not be treated as a clearance instrument.
It may help a user notice a strong active transmitter or inspect for an obvious camera reflection or magnet, but it cannot by itself establish that a home, boardroom or vehicle is either bugged or clear.