Assuming they were on someones radar screen for it to just suddenly go away screams catastrophic circumstances which occurred suddenly.
consider this though, flying through a volcanic ash cloud ( There are current eruptions in the area) could have eroded off various comm and transponder antennas to a point of rendering them incapable of transmitting.
something to think about
It would have been dark... and ash does not show up on radar. The first indication of entering it would likely be static discharges and st elmos fire
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