Walter
The missile has three stages of propulsion. Boost (get the missile off the launcher and up to flying speed) Sustain (keep the missile at flying speed for as long as possible) and Coast (missile keeps going until drag / gravity brings reduces velocity below a speed that the missile can fly). The missile was most likely in the final stage of its flight when it intercepted the 777, thus no fire at the back to give a smoke trail. The video that is on the net of the start of the aircraft decent has all the hallmarks of a proximity fuzed warhead from a missile being the cause of the loss of the aircraft.