Often in oceanic areas there is not radar coverage. Aircraft are controlled via HF radio contact with the appropriate ATC agency, however it is often outside radar range. So yes, it is entirely possible they were not being watched on any radar.
As was discussed, many aircraft (such as AF447 mentioned earlier) are equipped with flight data monitoring info which is automatically uplinked via satellite communications. However I don't believe it is required, and this aircraft may not have had it.
As for cause, it is the who knows stage. Have at all the wild guesses, but much like the Air France accident, it may well end up being quite a while before we know what it was, and it could be something no one has even thought of yet.
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