Tax evasion is the German national sport.
I’d call it income evasion.
I was watching some German news piece along about 1998. German border folks had stopped this German couple in an RV driving over the bridge into Luxembourg. Went through the whole RV....they had a coffee can with 40,000 D-Marks (around $20k). Guy was a plumber and had weekend work where cash was paid. This was his half-year take. Apparently, the couple had an account there and hid the income.
From the trade and craftsmen, I’ll bet more than half of them have safes in their basement and easily have 100,000 Euro stashed away that never gets reported to the government for taxation purposes.