The larger public as well as the authorities know Tetra as ‘the governmental network’, call it ‘C2000’ in the Netherlands or ‘Airwave’ in the UK. That’s the reason why people probably react nervously when talking about Tetra and Tetra usage by civilians. It’s not commonly known that Tetra is just a digital communication standard like DMR, which also can be used for civilian purposes. The encryption makes the difference. Dutch authorities as well as governmental users in the surrounding countries overlay Tetra communication with strong encryption called TEA2. This encryption is under embargo, which means possession by non authorized users is prohibited by law. Which means this is something a radio amateur isn’t interested in.
On the other hand ‘open’ / non encrypted Tetra is also used for instance by larger companies. Harbor services, parcel services or city services use Tetra for their daily business. And exactly this is getting interesting for radio amateurs. Because those users have Tetra radios with ‘clear’ firmware, without options for encryption, which means this is also covered by the usual amateur radio licenses. In fact anyone owning a Tetra radio can listen in to the communication at a particular (amateur radio) frequency. When searching YouTube you’ll get numerous examples of listening into Tetra communication with the ‘RTL dongle’.
To be sure a Tetra radio has clear firmware, you should look at the sticker with the TEI and serial number. Units which have been delivered to civilian users (and never have been active in an encrypted network) have the word ‘clear’ printed on this sticker. For radio amateurs, but perhaps also for suspicious police constables this is the first indication that the set is not a ‘loan gift’ form the government, has never been active in a governmental network and will never be able to access such a network.