SkyNews reports have deliberately avoided using any reference to possible terrorist actions. Now say the plane issued a distress signal one hour AFTER it was due to land in Cairo. Sounds to me like the plane may have been diverted, the transponder turned off, and the passengers taken hostage. That doesn't explain reports of a "fireball" in the above comments, not sure where those have come from.
I’m reading elsewhere that the distress signal is likely the beacon from the downed plane in the ocean. But what do I know.