One additional comment related to Doppy's post above:
At the German-Polish border I remember the border guard saying something about the Volvo, and I showed him the rental documents and let him know (in Polish) that the Volvo was for some reason not included in the Hertz list of non-exportable cars. That was good enough for him. No questions (about the car at least) at any other borders (Slovakia, Hungary, Slovenia, Austria).