I have had documents checked flying back from Italy, Greece, Bulgaria, Spain and Romania. Each time they were checking everybody, not random people.

Our family has also gone through a border control checkpoint driving across the Czech / German border on the E50 / A6 but they didn't stop us.

My two older boys were recently on a Flixbus that crossed the Austrian / German border on the A8 / A3 and they were stopped and the whole bus checked. A middle-aged man who apparently had unclear visa status was held by the Bundespolizei, did not re-board the bus.