Comment by adrian_b

In general, it is very easy to detect that radio signals are present.

A better comparison is with radio signals for which a method of spread-spectrum modulation has been used, chosen such as to have a bandwidth so wide that the averaged signal falls below the thermal noise level.

Such radio signals will also not be detectable without special detectors.

WiFi and Bluetooth use spread-spectrum modulation methods but they have relatively low bandwidths, so they can be easily distinguished from thermal noise. Much wider bandwidths are required to prevent detection.