If it was minding its own business, how come it found itself within interception range of a P-51?!
Have you taken a look at the Straits of Malacca and how close Sumatra is to the Malayan Peninsula/Singapore?
During Confrontation, the Indons quite frequently ran their (civil) Convair 880s at high speed over Sumatra in towards Butterworth to test the RAAF Sabres'/RAF Javelins' reaction times, always turning back before the Sabres/Javelins were ever in a position to catch them (probably at the border?, which was pretty close). (Not that I think that either type ever could catch a Convair 880 that didn't want to be caught without a very good lead - the Convair was a very slick mover if its driver didn't mind watching the fuel quantity gauges dropping at the speed of a whore's drawers.) They didn't try it, as far as I know, after the Mirages arrived, but by then, things had cooled down appreciably politically and the RAAF Hercs were overflying Indonesia between Darwin and Changi rather than going the long way via Perth and Cocos Island to Butterworth, as they were forced to do during Confrontation.
I have to agree with others; no matter how draconian a security/national interest muzzle was applied by the PTB, I don't think the Butterworth knucks could have kept it to themselves if they'd splashed a P-51 - or anything else for that matter.