What a signal detector finds
Nearly every modern surveillance device transmits: a WiFi camera streaming footage, a 4G bug carrying audio, a GPS tracker reporting location. A signal detector sweeps the radio spectrum and alerts on any of them, which is why one device covers what would otherwise need three.
What it cannot find is anything not transmitting: a camera recording to a memory card, or a device switched off. For those, see lens detectors and non-linear junction detectors.
Consumer or professional
Entry-level units tell you a signal is present and roughly where. Professional units identify the frequency and type of transmission, which matters when you need to tell a hidden device from a neighbour's router. If you are sweeping regularly rather than once, the professional units save considerable time.
How to sweep a room for a listening device
Start by turning off everything of your own that transmits: phones on flight mode, laptops off WiFi, smart speakers unplugged. Each of those is a radio source, and a detector has no way to tell yours from anyone else's.
Then work the room slowly with the detector at low sensitivity, raising it until the unit responds to a known source such as a phone briefly taken off flight mode. That sets a useful baseline. Sweep close to surfaces rather than waving the unit around: signal strength falls away quickly with distance, and the difference between a hit and nothing is often a matter of centimetres.
Cover the places that have both power and a reason to be there: mains sockets and adaptors, power boards, desk lamps, smoke alarms, air vents, the underside of desks and tables, and anything that has appeared or moved since you were last in the room.
What a bug looks like in practice
Almost never a device that looks like surveillance equipment. In practice it is a mains adaptor, a USB charger, a power board, a desk clock, a pen left behind, or a small sealed box taped inside a cavity. The giveaway is rarely the object and usually the circumstance: something in the room that nobody can account for.
The limit worth understanding
A radio detector finds transmission. A device recording to internal memory for collection later transmits nothing at all, and no radio sweep will ever find it. Neither will a sweep find a transmitter that only sends in scheduled bursts, unless you happen to sweep during a burst.
That is why a proper check is a radio sweep plus a physical search, and why sweeping more than once at different times finds things a single pass misses. For devices that are switched off entirely, see non-linear junction detectors, which respond to the circuit board itself rather than to any signal.
Common questions
Will it react to my own WiFi? Yes, and that is expected. The skill is in narrowing down by moving through the room and watching signal strength.
Can it check a car? Yes. For underbody searching specifically, see GPS tracker detectors with a magnetic probe.