If an aircraft breaks up at flight/cruise altitude (in this case FL370), for whatever cause, and assuming this is what may have happened, inherent inertial forces and of course drag opposing the inertial direction and gravity acting in the vertical direction and external forces (i.e. winds aloft) act on an (unpowered) aircraft (or aircraft debris).
It appears that MSR804 was following a flight path including overflying RAPOS WP
and KUMBI WP
(reported to be the approximate last radar position of MSR804 at around 0030Z). Therefore looking at the flight path direction and coordinates being reported for the last radar contact position at approximately KUMBI WP
and the reported coordinates for the found/located ocean debris, it appears logical that the aircraft (or aircraft debris if truly an inflight breakup) continued on a forward inertial path while being displaced E by winds aloft (at the last position time, being roughly 280 Deg. True at 75 Knots at FL370 at both HECA and LGKP) while descending.
At a reported approximately 530 NM
/Hr ground speed at FL370 at the time of lost radar contact the aircraft (or debris) would have been moving over the ground (ocean) at around 9 NM
/ minute and then (unpowered) decelerating while descending, and along with the reported winds aloft acting almost directly from the West displacing the debris to the East. IMO the ocean debris location found around 60 NM
Left of the course at the reported last radar position seems logical relative to a potential inflight breakup (from whatever cause) at FL370.