British in Schengen zone: what if you don't have your passport?

The whole point of the Schengen agreement was to abolish systematic border checks between the participating state -- not by applying any particular "white privilege" to certain travelers, but simply by having the border crossings be unstaffed yet open by default. So the fact that you're not inspected when crossing between France and Germany is how the system is supposed to work.

In recent years several of the member states have "temporarily" introduced extraordinary checks at some (but not all) of their internal border crossings. But even so, the relevant question is not, "are you allowed to cross this border?", but "are you allowed to be present in France?" (or wherever the check is physically conducted). Your rights and duties are exactly the same whether police stops you 20 meters from the border to Germany or on the street in Paris.

What actually happens if you don't have your passport depends on national law and is not regulated at the EU/Schengen level. So only a few general things can be said:

• You may be detained while the police attempts to find out your identity.

• If the stop happens right next to the border, the police might informally offer to lose interest in your identity (so you avoid detention) if they see go back to Germany.

• In countries that require of their own citizens to carry ID at all times, you may be subject to the same penalties as forgetful citizens are. Note that a driver's license is usually not sufficient for satisfying such a requirement.

• Otherwise the hassle of being detained while your identity is ascertained should be the worst outcome. As long as you can claim to be a Union citizen, the freedom-of-movement rules oblige countries to give you "every reasonable opportunity" to get hold of documentation or have someone bring it to you, before they conclude you're illegally present and remove you from their territory.

• For non-EU/EEA citizens (which may become relevant after Brexit) this protection would not apply, and a country could, according to its national law, levy fines that don't apply to its own citizens, or decide that not having documentation with you constitutes sufficient proof of illegal presence to eject you forcefully.