To add: when our resident dachshund was elderly, sick, and getting close to death, she looked ragged and emaciated too, despite all our care. She ended up dying of a spine infection, so she wouldn’t have been able to go someplace quiet to pass on, but other animals we’ve kept did pull disappearing acts shortly before death.
This happened with my boyhood dog. She was quite elderly and slipped her leash after being let outside on Halloween night when I was in college. We never saw her again. One of the saddest nights of my life.
There’s no excuse for abandoning an animal. There are too many good options for placement these days. But sometimes animals do run away to die.