[Leendert]

Laurent,

In google.books I found the "Report of the International Committee of the Red Cross on its activities during the Second World War, Sept. 1, 1939- June 30, 1947".

It says that after the Embla incident "the British government requested that, as an additional precaution, ICRC vessels should give their position by wireless to permit their identification etc..."
This meant the cooperation of the Germans as well.

If the measure was additional, then whatever neutrality markings of the ship had not been sufficient enough?

The report does say that the ICRC "notified the Powers of these violations", also referring to the attack on the vessel Cristina on 6 May 1944.

Regards,

Leendert