Magnetic detection
I used to be crew member on an ASW aircraft (P-3) which was equipped with magnetic anomaly detection - MAD, and I used to maintain the MAD equipment. The device was made as sensitive as possible. It could detect trucks going by on a highway near our runway - when we were within 100 meters or so, or it could detect a shallow sub directly under the aircraft. This short range was useful to us as it indicated when we were directly over a submarine. The MAD operator would yell "MADMAN" when he got a hit. They are still used for ASW.
A submarine has a very large amount of ferromagnetic material compared to a 777, and to be detected, a submarine had to be relatively shallow compared to the S Indian Ocean depths.
Given its limited range, I doubt that MAD technology would be of much use in this search. We didn't use it for searching, after all, only for precise locating, and we were hunting much larger prey. I'm sure the folks involved have thought about all this, and probably consulted specialists.