Who notices the scar that gives away Odysseus's identity?

We have reached Book 19 of The Odyssey, and our hero has finally returned home to Ithaca after his long, epic voyage. But Odysseus remains disguised as a beggar. He does not want to reveal his true identity just yet; he does not want Penelope's suitors to get wind of his arrival. Odysseus will not even reveal himself to Penelope, and, like everyone else, she does not recognize her husband: she thinks he is a beggar. She provides him with generous hospitality—a common theme throughout the poem—and instructs her old housekeeper Euryclea to wash the stranger's feet.
Euryclea also used to be Odysseus's nurse when he was a boy; she has bathed him numerous times. As she mixes the water for Odysseus's feet, Euryclea remarks on how similar the stranger looks to her master. It is only a matter of time before Odysseus's true identity is discovered. Despite Odysseus's best efforts to hide his scar, as soon as Euryclea touches it, she knows it is him straight away. The old nurse is so overwhelmed with joy that she allows Odysseus's foot to drop into the basin, spilling water everywhere. Odysseus's identity has finally been revealed, but for now, he swears Euryclea to secrecy. He still has to settle accounts with the suitors, and word must not get out that he is home.