The US Naval Institute reports that ladders in a speedboat were one indicator that led to interception near Malaysia:

The ship was responding to reports of pirate activity and heading to the scene while sending it’s embarked Royal Danish Air Force MH-60R helicopter in advance to observe the area, according to a Thursday news release from the Danish Armed Forces. The helicopter sighted a speedboat that afternoon with eight men on board in the vicinity of merchant ships in the area and observed that the boat was carrying a number of piracy-associated tools, including ladders.

By the evening, Esbern Snare was close enough to launch rigid-hulled inflatable boats (RHIBs) carrying Danish naval special forces personnel and called on the boat to halt and permit boarding, the news release said. When the boat refused to respond to the call, warning shots were fired, with the pirates responding by firing directly at the personnel in the RHIBs. A brief firefight then ensued, in which no Danish personnel were hit but five pirates were shot, with four of them killed and one wounded. The motorboat sank after the firefight and the surviving four pirates and the bodies of the dead pirates were taken aboard the frigate, where the wounded pirate was given medical treatment. The release said that Denmark’s inter-ministerial working group will handle what will happen next to the pirates.

Unregulated seas and collapse of safe markets generally is the root cause of piracy in the modern age. Someone financed a speedboat and ladders, let alone weapons.