jcjeant
Jean-Paul Troadec has responded appropriately to the question asked. He did mention that the area in which the debris was found was originally searched (US Navy TPL towed by "Fairmount Glacier") and nothing was heard. That was during Phase 2 and between then and Phase 3, the BEA engaged the Drift Group to try and narrow down the area to search.
The group came up with a area of probability that was to the NNW and around 30NM from the LKP and encompassed around 2,000 ^2 km, which was searched using towed side-scan sonar and AUVs equipped with SSS. Phase 3 was extended and the areas to the west and south of those already searched were covered by AUVs. There was an unfortunate interlude which lead the search to be moved over 40NM to the West by South of the LKP and this deviation was responsible for the debris not being found during the extended Phase 3 search.
Following Phase 3, the Metron Report made a statistical analysis of all the data they had at their disposal and concluded that the aircraft would be found within 20NM of the LKP.
Phase 4 under the operational control of WHOI used 3 AUVs and resumed searching from where they had left off at the end of Phase 3, ie about 10NM NNW of the LKP. Working methodically through the remaining area north of the LKP, it took them about one week to come across the debris.
The reason for neither of the pingers being heard remains to be found, and as far as I know, there has been no report of the FDR's missing ULB being located. I am sure that that line of enquiry will be run in conjunction with the ongoing FDR/CVR data analysis.
Overall, the initial air/sea searches for floating debris failed badly, but the ongoing planned secondary searches for the ULB pingers and then using alternative methods was done well.