Subversive art project idea:<p>A RaspberryPi(or similar) with a $12 TV tuner software defined radio, and a small database of the various frequencies and bands particular flavours of your local LEO and&#x2F;or federal TLA use.<p>I know our local highway patrol, for example, use at least: P25, police issued cell phones, iPads with cellular data, HF radio (at least on rural&#x2F;country cars), and whatever they use for the data connection for their in-car computers.<p>Most&#x2F;all of that would be encrypted, but hey - metadata&#x27;s a free-for-all according to them, right? I bet if you knew what to look for, you could quite reasonable detect the presence of a highway patrol car when you saw some digital P25, a cellular connection on the bands used by the contracted cellular provider, and I&#x27;d bet good money that patterns of encrypted data that things like ALPR systems send would be quite distinctive...<p>That&#x27;s all staying pretty solidly at the whitehat end of the wide grey space of &quot;what&#x27;s allowed&quot;. If you were much closer to the blackhat end of that spectrum, I&#x27;d also bet pretty good money that sophisticated criminals are already exploiting GSM and 3&#x2F;4G weaknesses to individually identify cellular radios by cracking the encryption to see the IMSI data. And that&#x27;d all be passive and undetectable, if you were firmly in the blackhat camp, active attacks against their P25 radios can (as pointed out) be done for effectively zero dollars.