3 ideas
The detectives treat the position of the corpse as a sign to be decoded, but the solutions they reach depend on Vogelstein's account of how the body lay. When he changes that description, the reading and the implied culprit change with it, so the evidence being interpreted is not fixed.
The account reaches the reader through Vogelstein's written report to Borges. Borges's reply exposes that Vogelstein lied and altered his descriptions of what he saw, which forces the reader to reinterpret the earlier narrative.
The 'eternal orangutans' combine two ideas. One is the infinite monkey image, in which random typing given unlimited time eventually produces any text. The other is the orangutan killer in Poe's 'The Murders in the Rue Morgue.' Together they raise a question the novel keeps asking of its clues: are the apparent patterns meaningful authorship or the accidental output of chance, and can a reader ever reliably tell the difference?