[Tailgunner Joe]

BUK Capability - Although it has it own Identification Friend or Foe (IFF) system, this is only able to establish whether the target being tracked is a friendly aircraft. It is the electronic equivalent of a sentry calling out "Who goes there?” If there is no reply, all you know is that it is not one of your own side's combat aircraft. It would not give you a warning that you were tracking an airliner.

[Viennacon]

But a commercial aircraft would reply, would it not? They communicate with the transponders aboard aircraft, so if the transponder was on, it would respond to such an interrogation. Encountering SAMs cannot be a strange occurrence to commerical aircraft, I would guess the plane was interrogated by other Ukrainian air defense systems as it passed over the country.

[Viennacon]

If it did not have information on any commercial flights approaching, then that raises the question of why it fired on this particular flight. It cannot have been the first that passed over the area.