Just some additional info, to help the brainstorming here...
Contrary to the RPC 1.4 or 0.7 degree beam, its IFF system has a whooping 10 degree "pencil" beam!!!
If the target is inside of it, giving "I'm your target" signal, the crew could accidentally lock on target, 60km away from the actual IFF signal sender.
This is why own fighters should stay far away of any S-200 engagement.
(See piston table above.)
WARPACT procedures called several IFF checks during an engagement.
1st check after target acquisition.
2nd check just before missile launch.
3rd check during missile flight.
My guess is the crew did several IFF checks on the passenger plane, just the drone was inside of the 10 degree area...