When Hans is reading a copy of the 'Frankfurter Zeitung' in the Colonel's office, the paper is quite clearly a mock up. Although the 'Frankfurter Zeitung' was a German wartime newspaper, there is an advert at the foot of the back page for what appears to be the early to mid-80's model BMW 3 series.
In the final scene, when dirt falls on Herr Otto Flick's head after the coffin explodes, Private Helga Geerhart fails to hide her obvious 'out of character' grin at his misfortune, even looking away to her left to be more discreet.
If you wish to keep your identity and allegiance a secret, "Nighthawk" (René) may be a good call sign/cover name for radio transmissions, but "London" (British Intelligence) not so much.