A short tutorial, from a 35 year controller.
In aviation, there are two types of radar, 'primary' and 'secondary'. In today's civil environment, primary is seldom used, as it is unreliable and costly to maintain.
Primary radar is a display of transmitted energy that has been reflected back to the antenna. It appears as a 'blip' or 'blob', with some variation in brightness. It has no altitude information. A small target at short range would appear to be the same size as a large target at moderate range. Military radars are quite powerful primary radars, as only a kamakazi would fly into combat with a transponder.
Secondary radar consists of a ground transmitter sending a radio signal out, asking "hey, anybody out there?". The aircraft transponder receives the signal and replies with a four digit code and his altitude. The ground receiver then, via computer, displays the target on the display in the correct position, along with the altitude.
The aircraft transponder is also part of the collision avoidance system, but that is part of another tutorial.
So the military class radar would have known exactly what happened to this plane.
And that it didn’t automatically ‘explode’ when the transponder ceased to transmit. Ie, they didn’t see the pieces on radar as they fell thousands of feet to the (presumably) ocean.
We may never know exactly what happened. Looking for it in the Andaman Sea is indication that whatever happened likely wasn’t ‘sudden’.