HNGN, April 28, 2015 reported: "Our sensors detected something, an uncertain underwater object, at lunchtime on Monday April 27, 2015] and then again overnight," Captain Olavi Jantunen told Helsingin Sanomat newspaper, reported the Guardian. "Three [grenade] charges were dropped, but they were warning charges, with a light burst, meant to warn rather than do any harm to the object."
BACKGROUND AND COMMENT
The (probably) Russian mini-submarine or large diameter unmanned undersea vehicle (LDUUV) may be there to "trip" or test Finnish sensors or to make Finland politically nervous. This is like the Russian aircraft incursions - part of Putin's confrontational style. The mini-submarine may be a development of Russia's Losos-Pirana.
The Finnish Navy would have determined very quickly - via tell-tale sensor "signatures" - what type of undersea craft has trespassed Finland's territorial waters. For diplomatic-political reasons Finland would not accuse the trespasser, particularly if it is powerful and combative Russia. Though part of the European Union Finland maintains a careful neutral policy.
The "sensors" used to alert the Finnish Navy may be hydrophones, magnetic anomally and other wireless or fixed sea-floor arrays deployed inside Finland's territorial waters. Nokia Data Systems equipped the Finnish military with an integrated coastal surveillance system - according to a late 1990s report. This system would now be very high-tech with many of the sensor uprgrades and increased computer processing power available today. Even quite a small mini-submarine or LDUUV might be detected. Sweden's has its own Malsten Station array which would most probably have been much improved since the 1980s.
The grenades used may be Saab ASW-601s [right-click mouse to translate into English] fitted to multiple grenade launchers on such Finnish vessels as the Hamina class missile boats. These grenades may have been aimed away from the suspected submarine/LDUUV or have adjustable burst features.
See Submarine Matters reports of October 25, 2015 and January 15, 2015 on Sweden's late 2014-early 2015 experience of suspected Russian mini-submarines or LDUUVs.