While abandoning sick pets is utterly reprehensible, it should be mentioned that sometimes sick animals will creep away on their own to die. For all you know, some family was frantically searching for their dying longtime friend so she could be comfortable in her last hours.
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.
We have a local internet forum called ‘Front Porch’ where people post that type of information. She wasn’t on that. Our local grocery and post office also has a bulletin board where folks post for lost pets; when I went down there after she passed away I didn’t see anything posted in either place.
We had a golden lab and when his time came he found the place he wanted to die and laid down. We hunted everywhere for a couple of days and finally found him right before he passed away. Every dog I’ve had that died of old age usually looked pretty bad at the end with weight loss even though they were getting all they wanted to eat.