A paint job and a knowledgeable pilot who can fake it well enough to disguise the airplane as a legitimate flight long enough to get close to an intended target is all it will take.
Even if it raises suspicions no one is going to shoot down a passenger airliner until/unless there is certainty that it is a hostile.
By that time it will probably be too late.
“A paint job “
As a FReepeer posted yesterday, beware of any 777 with an Earl Scheib paint job.
Out of radar range just fly up under a legitimate scheduled flight, remaining a hundred feet or so directly below it. Monitor that aircraft’s comm with ATC to anticipate changes in assigned altitude & heading so that you are prepared to mimic turns and climbs/descents.
Do it at night and the paint job will not matter. Do it during daylight and it will not matter until you are too close for anyone (outside of a military target with close-in air defense) to do anything about you.
Time to watch container ships.
Didn't China shoot down a passenger jet a few years go?