Three ways to find a device
Most hidden devices transmit, and a signal detector finds anything broadcasting on 4G, WiFi, Bluetooth or radio, covering the majority of cameras, trackers and bugs in one sweep. But a camera recording to a memory card does not transmit, so a signal detector will not find it. That is what a lens detector is for: it flashes light and finds the reflection from a lens, transmitting or not.
Devices that are switched off entirely defeat both methods. Non-linear junction detectors find electronics by their circuitry rather than their signal, which is why they are used professionally and why they cost what they do.
Preventing rather than detecting
Microphone blockers take a different approach: they emit ultrasonic sound that microphones pick up but people do not, so any recorder in the room captures unusable audio. Useful for boardrooms and private meetings where sweeping before every conversation is impractical.
Start with what you are actually looking for
The four things people search for are cameras, microphones, trackers and phone tampering, and no single device covers all four well. Deciding which one you are worried about narrows the equipment quickly.
If it is a room you sleep or change in, a camera is the concern, and an optical lens finder is the tool, because it works whether the camera is transmitting or quietly writing to a card. If it is a conversation, a microphone is the concern and a radio detector is the tool. If it is your movements, a tracker is the concern and the search is physical as much as electronic. See spy camera detectors, listening device detectors and GPS tracker detectors for each.
Why one sweep is rarely enough
Almost everything covert spends most of its time silent. A tracker reports on a schedule. A transmitter sends in bursts. A camera recording to a card never transmits at all. A radio detector can only find something during the moment it is talking, which means a single pass through a quiet room proves very little.
Sweeping at different times of day, and while a vehicle is actually moving, finds things a one-off check walks past. So does simply opening the objects that could hold a device, which no handheld unit can do for you.
Clear the room of your own signals first
The most common reason a detector appears to find something is that it has found your phone, your laptop, your smart speaker or your neighbour's router. Put your own devices in flight mode or out of the room, then set the sensitivity so the unit responds to a known source and nothing else. Without that baseline you are reading noise.
When equipment is the wrong answer
Detection equipment suits a precaution, a hotel room, a rental, a periodic check of an office. If you have a specific, credible reason to believe you are being monitored, the honest answer is that a handheld unit is not the right tool and the situation warrants professional help or police involvement rather than a purchase.
Common questions
Which should I start with? A combined signal and lens detector covers the most ground for the money.
Can I check a car for a tracker myself? Yes. GPS tracker detectors with a magnetic probe are made for searching wheel arches and underbody.