Is it possible I added it and forgot? I highly doubt that. I only add mangas without chapters if it's from a related series, or by a artist who's works I like. There's only one fairly new series this artist is currently working on that I just added the other day, and I haven't even viewed or added any of their other works.
This wouldn't be the first time either I find something suddenly added to my feed. The "test" manga was suddenly added to my feed a couple of years ago as well.