The more I think about this, the more I wonder if it's as simple as this:
Phase 2 = side-scan sonar survey
Phase 3 = identify and further investigate targets based on Phase 2 survey.
They were discussing Phase 3 not long after Phase 2 started; perhaps Phase 2 was never expected to result in locating the aircraft without more work. Given all the false sightings of floating debris, I couldn't blame them for keeping mum now even if they had a good idea that they could see the airplane in the sonar image.