For months I've been having this issue. I rarely add songs to my favorites playlist but lately, every time I log on there is a new song in there. I'm 100 percent sure I didn't add it myself. It's always a song I don't know, esometimes even 2 and it is always one that isn't in my own library. It doesn't matter all that much but it does get annoying to have to delete a song from my list every time I open it.