You can just do a foia request for that. Just put in very specific language, like "any and all communications containing the words "nr" "ft" "r****d"" and that will get them paranoid af.
They use an open frequency, not the cell network. You can use an inexpensive rtl-sdr to monitor them. People do it all the time to integrate their meter with home assistant.
It's no different than looking at someone's WiFi network name.
Investigation of what exactly? There's nothing illegal about it if you don't make any threats. You could follow someone around and take pictures and notes on them too, not illegal.
That isn't piracy. It's time shifting, and it's perfectly legal to do it.