How does the author use foreshadowing in Poirot Investigates?

In Poirot's ninth adventure, Poirot and Hastings entertain a conversation with their friend Dr Hawker. They rush to see Dr Hawker's patient, Count Foscatini upon finding out from his housekeeper that he called asking for help. They find him dead in his chair with three empty plates and half empty coffee cups. Poirot discovers all the important details that give him clues to follow. These clues foreshadow later developments but they are only meaningful to the detective. Despite the presence of the police or his friend Hastings they are unable to read the clues in the same way Poirot can. Only he can see that Count Foscatini's teeth have no dark stains from coffee, despite the fact that his coffee cup is half empty. Only he is perplexed that the curtains are not drawn or deserts not finished after the dinner with his two guests. The events progress, indicating that all is what seems to appear. His two guests didn't murder him despite the circumstances pointing that way and Poirot knows it.