What does Joe mistakenly leave behind at Peach's beach house in the novel, You?

Joe leaves a mug of dried up urine in Peach’s beach house. It isn't until he is nearly back to New York that he realizes he left the mug, which contains his DNA behind. Ultimately, he decides not to worry about it since Peach was probably in the habit of giving the key to her house to a variety of people, any of whom might be suspects.