Intermittent operation
Some devices may transmit only at selected times or under particular conditions. A quiet spectrum at one instant is therefore not proof that a location is clear.
Yes, some surveillance can still be investigated when no obvious RF transmission is present — but it requires more than a simple RF detector.
A device that is not transmitting may not present an obvious RF signal at the moment it is examined. That is why professional TSCM should not be reduced to watching a handheld RF detector for an alarm.
Some devices may transmit only at selected times or under particular conditions. A quiet spectrum at one instant is therefore not proof that a location is clear.
A recorder can capture information locally without continuously radiating an RF signal. Physical and electronic inspection remains important. This is one reason a home investigation should combine methods rather than rely on RF alone; see how hidden listening devices can be investigated at home.
Spectrum analysis, physical examination and other appropriate detection methods provide different pieces of evidence.