From a radar point of view...
All airforces surveillance worldwide is being done with primary radar. No enemy will switch on their transponder before they attack, so SSR is kind of useless from a military border-watching point of view.
30 years ago, a normal primary radar was able to follow flocks of birds without any problems at all. Todays radar equipment is of course much more refined and exact. Following a B777 is among the easiest tasks ever. Its massive, not supersonic, no jamming, and it oftens flies quite high and straight.
So, either the normal 24/7 radar surveillance in the area was not operational at this time (how embarrasing for all involved countries), or there are quite a few people within the borderwatching units who has known all the time where, exactly, primary radar contact was lost...