I read a fascinating but scary Economist article titled Peril on the Sea. It presents classic examples of "indicators" that can be used to formulate intelligence "warnings" for decision-makers. (Indications and warning is defined in the DOD Dictionary of Military and Associated Terms. Definitions are taken from the DOD Joint Electronic Library's Joint Publication 1-02 [.pdf].) From the Economist article:
"According to a new study ("Security in Maritime Transport: Risk Factors and Economic Impact" [.pdf, overview]) by Aegis Defence Services, a London defence and security consultancy, these attacks represent something altogether more sinister. The temporary hijacking of the Dewi Madrim was by terrorists learning to drive a ship, and the kidnapping (without any attempt to ransom the officers) was aimed at acquiring expertise to help the terrorists mount a maritime attack. In other words, attacks like that on the Dewi Madrim are the equivalent of the al-Qaeda hijackers who perpetrated the September 11th attacks going to flying school in Florida."