If I had to guess, they're using frequency hopping across your frequencies. Afaik, many emergency and LE services use a combination of encrypted and unencrypted radio comms, so you're probably only hearing the unencrypted comms as they hop into your frequency.

I doubt it's related to the backdoor references in the article. But it's been a while since I played around with RF and even when I did, I wasn't really an expert despite it being in the job description.