The most interesting thing is that to avoid the radar picking up the aircraft it would have to do one of these things:
1) Disintegrate
It's definitely possible to cause an explosion on board, but very hard to break the aircraft into pieces small enough to avoid creating a radar signal.
2) Descend Rapidly
By flying very low the aircraft could avoid detection, but from the altitude it is alleged to have disappeared at the descent would be noticeable, unless some strange quantum physics allowed it to teleport (in which case it's probably teleported to a far more interesting place than a few hundred feet below itself). Authorities have reported indications of a rapid descent, but this has not been confirmed.
3) Have Stealth Capabilities
Aircraft can be built to avoid radar detection, the 777 is not included in this list!
4) Actually Disappear
Right now, this seems to be a reasonable conclusion, after all nothing has been found to eliminate it or to lend credence to any other theories. Has anyone checked the location of Derren Brown at the time of the incident?
NB I wish everyone on board and their families well, it's clearly a very difficult time. However as it stands I believe it is possible that the aircraft and it's occupants are intact and well. This is certainly a very interesting incident.
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*Additional*
As for the fact that the authorities aren't saying much, it seems unusually quiet, but really they won't give too many details, there have been aircraft crashes before where components were stolen because the crash site was announced before the emergency services had set up an effective cordon. Likewise if it has been hijacked the last thing you want is civilians wandering around whatever obscure airport it's been landed at!