The pattern I have noticed is that non-white passengers (such as myself) get checked while white passengers don't; on the other hand sometimes waving a passport (without opening it) from a developed country is sufficient to get past regardless of ethnicity.
CDG occasionally does passport checks even when arriving from original Schengen members.
I've experienced spot passport checks on buses and trains at or near internal Schengen borders like DE/CZ, DE/DK, DE/NL. Traveling on a Canadian passport my interaction with border officials in such cases is less than five seconds. It's just a quick glance at the face and then the passport is handed back, no questions. I think it has less to do with ethnicity than presenting a passport from a country that is low risk in terms of migration. For example, Japan is usually included along with USA/Canada/Australia/New Zealand/UK as low risk.