How does the EU countries know if someone has overstayed when they do not enter/exit the external border/airport?
For example, if you travel by plane, you'll get stamped on your passport and maybe you will get register for your traveling in the Schegen information system (SIS).
However, if you travel by bus or train, I don't think you will get travelling register.
So how do they know if you overstayed?
Edit: I mean for a person to traveling within the Schengen Area to another country other than the country that they have a residence permit
Because a friend of mine (he was an EU resident) travel to another EU country by bus. And he said the Police just look at his residence permit and not taking note/stampped/register anything at all
Because EU resident are allowed to stay 90 days visa free within the Schegen area. So when they overstayed those 90 days, I'm curious how will the gornverment know?