Wait! It was not a typo. Serbia-Croatia is indeed a Schengen border (Croatia is in Schengen). My point was that there was anti-bus discrimination even at this low-security border.

At the supposed non-borders within the Schengen zone, police are increasingly present. Often they get on buses (and trains) just to check out the passengers, obviously looking for passengers with migrant profiles.

Two or three years ago I crossed the ultra-low-security Germany-Denmark border on a local bus. There was no border security but I overheard the driver making an intentional phone call to someone to say that he had a foreign tourist aboard. Schengen has not completely abolished borders, alas.