S. of Japan E of Philippines
Email from a friend with more than 40 yrs of airline stick time in Boeings + Airbuses + other birds as well. Also accident investigator. My translation from another language:
"Hypoxia is not to be neglected....Seems now confirmed that ACARS pinged for more than 4 hrs... one could reasonably imagine that the plane flew until fuel starvation... believe search area should be S. of Japan, East of Philippines. (FYI: if XDR code change is halted in mid-operationm, the transponder blocks and is inop.)