Dozens of suspect ships travel in Dutch waters every month ‘under the radar’ without officials knowing where they have been, the Algemeen Dagblad reports on Thursday.

The vessels in question change their registration numbers or temporarily disable their GPS monitors when they are in areas known for the presence of terrorists or drugs or people smuggling operations, the paper said.

The AD said its information came from an analysis it commissioned from maritime information company Winward. ‘Such movements can be an indication of illegal activities,’ said Ami Daniel, director of the Israeli firm.

The data revealed that in the past two months 75 ocean-going vessels entered Dutch waters with defective registration numbers, possibly altered to conceal earlier voyages.

And prior to entering Dutch waters, more than 480 ships in the same period had switched off their GPS monitors for 24 hours or more for undisclosed reasons. This occurred largely in the Mediterranean Sea, in areas known for the presence of IS troops or where many refugees are trying to reach Europe. But it could also indicate drugs or weapons smuggling, the AD said.

Ships entering Dutch ports are asked which ports they have last visited, but GPS systems are not investigated. The national coordinator for terrorism and safety NCVT admits ‘there is a problem’, but does not include these incidents in their quarterly ‘threat profile’.