Just a thought.... what if the following plane were to tuck in a couple of hundred feet below the lead plane and almost directly underneath... the pilot in the following plane could see the lead plane but not vice versa. And the radar blips might be much closer together -- perhaps leading an operator to assume a glitch. Also, the following plane wouldn't be affected by the wake turb of the lead plane, as well. And if the following plane were flying with all nav lights off directly underneath and slightly back, I don't think its very likely any passengers would spot it, either.
Assuming radar has fine enough resolution to differentiate different sizes and classes of aircraft (as I think may probably be so), if one were viewing a screen and saw two returns, both identical 777 aircraft almost on top of each other, who wouldn't think that it was just a glitch?
James T Kirk used a very similar trick to confuse the Romulans into thinking the Enterprise was a "sensor echo" (episode: Balance of Terror).
Apparently 23th century sensors haven't improved all that much from 20th century RADARs.
I heard one very random comment on the news about a NZ person on an oil rig stating he may have seen the plane. Sounds like he took notice of the plane given it did not appear to be a normal situation and offered up some pretty decent details regarding his location and where he could see it.
The comment about fire makes you wonder how on earth could a big airliner catch fire with nothing being reported back from the pilots? I guess they could have been incapacitated quickly with no time to issue a distress message but in a manner that the plane stayed air born? Man this thing just gets more strange every day.
Ask Payne Stewart and his flight crew how that could happen.