I'm born here and lived here all my life and this has never happened to me.
Unreported work/income is estimated at about 10% of GDP: https://de.statista.com/statistik/daten/studie/20063/umfrage...
I'm not from Germany, and do have a foreign name, I guess that may play a role.
That's a good point I hadn't considered.
But it's not "you have to pay more". More like "just so you know, I have access to this internal tool, you pay me now and this thing that isn't supposed to be possible due to your situation is now done".