A growing number of so-called 'ghost ships' are turning off identification systems to engage in suspected criminal activities.
Analysts say cargo ships and other vessels are routinely "going dark", making it impossible for authorities to know where they are or what they are doing, risking collisions and loss of life in busy shipping lanes.
It is feared those craft are exploiting the vulnerability of maritime security to deal in drugs, arms or people, and that terror groups are among those benefiting.
A ship's automatic identification system (AIS) is the tracking device that charts every mile of its journey and is meant by to stay on throughout the voyage.
Under maritime law, passenger ships and non-passenger ships over 3,000 gross tonnes on international voyages must be fitted with them.
Ships deactivating the GPS device are breaking the law, as well as becoming a danger to themselves and other vessels.
Ami Daniel, chief executive of maritime analytics company Windward, told The Times: "This behaviour we would describe as anomalous and can be an indicator of illicit activity."
Windward analysts tracked one ship that had 12 days of questionable manoeuvres while sailing to Gibraltar, as it drifted off course and deactivated its tracking systems.
Later it "went dark" for 28 hours inside the area of the Port of Oran in Algeria, a region notorious for the trading of arms, drugs and people.
The data firm said it was extremely dangerous to turn off AIS in the middle of such a busy shipping strait.
Next, it spent 11 hours off the coast of Islay, the southernmost island of the Inner Hebrides, where there are no major ports.
Another vessel, an oil tanker, made two unusual stops, sailing untracked for seven and 15 hours respectively, while voyaging from Libya to Greece.
Such unscheduled and seemingly random stops suggest they were meeting other ships to make an illicit exchange.
Windward's data showed that 2,850 ships disappeared off the radar before entering European waters in January and February alone, and while some may have had technical difficulties, most are thought to have been engaged in illegal activity
Many of the dubious stops are centred on terrorism hotspots such as Libya and Syria, sparking fears terror groups such as Islamic State and al-Qaeda are among those exploiting the loopholes in maritime security.
While the threat to Britain is diffcult to guage, the UK Coastguard says it is taking seriously claims that hundreds of 'ghost' ships have been travelling to British waters.
More than 550 tons of drugs have been seized from 140 ships in EU waters since 2007.