My thing is wont passenger cell phones also be pinging towers if they had ever gotten close enough to towers to be picked up. You would have to know a flight path through all cell phone dead zones. Granted it is flying in SE Asia so id imagine there are deadzones everywhere.
I imagine that if someone did this intentionally and flew under radar to avoid detection, they'd probably try to round up passengers cell phones before hand and take the batteries out.
If one of the passengers was clever enough to conceal one, the people searching for the plane sure as heck wouldn't say "well we're actually still getting a cell signal from one of the passengers...." as the hijackers would look for it to disable it.
I think its entirely possible one of the cell towers got a ping, but they're keeping that info concealed.
What will be interesting is if any of the family members of the passengers on board have any kind of remote locators like "find my phone" or similar, and if that data gets out to the public.