Enzo was alone and locked in the house by accident. Because she was so ill, Eve neglected to think about Enzo's welfare when she left the house with Zoe (her daughter).
According to the text, Eve had awoken the morning of Denny's race with a terrible headache. She had vomited profusely into the sink and complained of pressure against her skull. Panicked at this new development in her illness, Eve had then packed a bag with clothes for herself and Zoe before leaving for her parents' home.
For his part, Enzo understood why Eve forgot all about him. He reasoned that Eve's illness had affected her judgment. To survive, Enzo drank water from the toilet bowl and nibbled on stray scraps of food around the house. Because he could not leave the house, Enzo carefully made sure that he only urinated and defecated on the mat by the back door. After three days and two nights, Denny returned home. Although he was furious when he found Enzo alone, Denny eventually came to realize how ill Eve was.