The Wall Street Journal says this is the latest theory:
U.S. investigators suspect that Malaysia Airlines Flight 370 stayed in the air for about four hours past the time it reached its last confirmed location, according to two people familiar with the details, raising the possibility that the plane could have flown on for hundreds of additional miles under conditions that remain murky.
Aviation investigators and national security officials believe the plane flew for a total of five hours, based on data automatically downloaded and sent to the ground from the Boeing Co. 777's engines as part of a routine maintenance and monitoring program.
If true, the plane could have reached all kinds of places, including "the Indian Ocean, the border of Pakistan or even the Arabian Sea."
UPDATE: Malaysian authorities say the Journal report is "inaccurate."