The TETRA standard is used in radios worldwide. Security researchers have found multiple vulnerabilities in the underlying cryptography and its implementation, including issues that allow for the decryption of traffic.
We use the same licensed VHF network at work for the last 30 years. Ever since our local cops switched to 'digital', 'encrypted' radio comms, we regularly pick them up on our VHFs across the entire district, regardless of what frequency we are on.
It's like their whole thing is leaking across the bands, which should not be possible given that it's encrypted but here we are. If it were anyone else, we'd be pursuing them for abusing the frequency band that we pay for but it doesn't feel like a battle that we want to start.
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.
Just out of curiosity, when you say you "regularly pick them up", is it intelligible audio (clear voice conversations) or is it the digital modulation that's leaking over to your freqs? I don't have a good answer for the first problem since that shouldn't be possible unless there's an open patch somewhere that shouldn't be enabled, but I can 100% believe the second problem. In either case, it's poor practice from the radio maintainer of the offending system... and it's illegal regardless since their license is not valid for out-of-band transmissions and the modulation format is probably not licensed for use on those freqs. You'd be surprised, but FCC enforcement will take that very seriously.