when optimus is doing the story behind the war on TFTM it shows megatron on a mountain ? top and he chucks a eneron spear at a TF out in the distance that gets hit and staggers then falls....

my idea is that after the cube got deep sixed to space...the fallen(took off after the cube, kinda like the if you find the cube you find megatron) and maybe crashed and burned into the earth and got knocked into stasis lock somewhere in egypt..hell maybe the ancient egyptians found and stuck him in a pyramid as a symbol of worship from the gods....

just a idea anyway....