Sarabi had been devastated when Scar had told the pride of Mufasa and Simba's deaths. To lose one was awful enough, but to lose both at the same time -- Sarabi's heart was just a well of pain and sorrow and blackest grief. With that grief came disbelief. It couldn't be true -- it just couldn't -- even though Sarabi knew it must be.
But Zazu confided in her that he had somehow lost consciousness in the gorge before Mufasa died, Sarabi began to feel the worms of suspicion burrowing beneath her skull, and when Scar and the hyena made it clear that the pride was to stay at Pride Rock and 'avoid the tragic airs' of the gorge, those feelings were cemented.
Sneaking out during the blackest part of the night, Sarabi made her way down into the gorge. She found her beloved's body soon easily enough, but not her son's. As she foundered in confusion, the lioness spotted something else that was wrong down there -- Mufasa's paws showed bloody gouges. Her eyes narrowed. Those marks could only come from claws, not from hooves or crushing. Cold fire sprang up in Sarabi's eyes and she cast a glance of fury and deepest hate back towards Pride Rock. Scar had killed Mufasa. He had taken her love, her other half, and perhaps her son.
The lioness who made her way back to the pride's rocky home was not the same as the one who had left -- this one wanted blood, and only blood would sate her now. One way or another, Scar would tell her the truth, and he would tell her what had become of Simba. But most of all he would pay for that which he had stolen from her.
One way or another.
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So. I always wanted to know why no one ever went down into the gorge (especially Sarabi) to mourn or anything, because if they had they would have spotted that Simba's body was missing. They just took Scar's word for everything. I suppose Scar could have forbidden anyone from going, but that would be very suspicious in and of itself. Super-weak point in an otherwise good storyline, in my opinion.