Isn't Malaysia extremely tight on security, along with Beijing bound aircraft checked NSA style by the Chinese Government? I'm no expert in Security but Aviation Terror incidents are not easy, but not impossible for those flights. More importantly why would a terrorist target that city pair.
It is intriguing that the aircraft has been involved with a structural incident In the past; but I'm pretty sure that part of the 777 wing is just aerodynamic and not mainly structural integrity. I suspect wing box and spars were replaced in any repair job. G-BNLL had similar damaged and flew happily - before doing it again.
The fact Boeing tweeted waiting for news reports suggests even they don't know what's entirely happened; suggestive of something happening very quickly with no trace.