Likely in the ocean. Pilots would have had time to make a distress had it been terrorists, An insane pilot/co-pilot is plausible, but this scenario brings up a few problems. If drama/conflict had occurred it is difficult to believe someone on the plane wouldn't have gotten off a phone call before the plane was flown into the sea or somewhere else. Aside from that a catastrophic breakup iin the air s the other likely scenario, which would explain the lack of an alert. This is a state of the art plane with lots of capabilities.
THe latest news, I think from the Malaysian military, indicates the plane was changing course. Interesting case.