A Dutch broadcasting company has sent a tracker by post to a Dutch warship in the eastern Mediterranean, allowing its movements to be followed.

TV and radio station Omroep Gelderland sent the tracker in an envelope with a postcard via the military postal service, which allows members of the armed forces to keep in touch with home. Officials check post with an X-ray machine to look for banned or dangerous substances but do not open items.

The broadcaster’s tracker passed through the checks unhindered, and Omroep Gelderland was then able to follow the frigate’s movements when it left Heraklion port in Crete.

A defence ministry spokesman has now told news website Nu.nl that checks on military post would be stepped up. Defence minister Dilan Yesilgöz has also been briefed.

The spokesman also said that the frigate’s whereabouts are currently not secret and its automatic identification system is switched on, allowing it to be followed by public tracking sites.

The boat was sent to the eastern Mediterranean at the end of March to support the French aircraft carrier Charles de Gaulle and is due to stay in the region until early May.