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.
If it had happened earlier that day, they might not have had time to put up notices. In my experience, when an animal sneaks off to die, it does so only a few hours to maybe a day before the actual death. Especially if the dog had already been sick for some time, they might have focused first on actually trying to find her themselves before putting out notices. I’m not saying that’s definitely what happened, but I felt it was at least possible and so mentioned it.
You could be entirely right, that the owners were heartless and dumped her when she became severely ill. Perhaps you’ll eventually find out. Did she have a collar, or maybe a microchip?