This incident echos the Air France disaster in 2009 where the plane en route from Brazil dropped out of the sky without a trace into the Atlantic Ocean. It took 5 days to find the wreckage and two years to locate the black boxes from the seabed. The recovery services had to work on a 35 mile radius to find them as they didn't know how fast the plane hit the water and debris were discovered over 25 miles away. The black boxes only seem to send out a distress signal if the aircraft reported a fault automatically or if the crew manually sent out a distress signal. I don't think they can be traced in most instances if a plane goes off radar due to weather interference or deep water despite them being able to withstand the hottest temperatures. Recovery teams can only deduce by estimation the location of a 'downed' plane through the last transmission broadcast and it's average speed at the time.
I remember a flight going from Kuala Lumpur to China a few years ago and it took 3 years to find the wreck and recover the black boxes. In this day and age and with all the technology available and in most cases; more superior tech, I still can't believe the black box aspect is so backwards.