Scarlet Brotherhood notes are found in Tirisfal Glades if you done the BFA launch event where Lordaeron City gets blighted by Sylvanas. In the little notes you find around the glades they mention that they are holding the true heir and lost Menethil. They claim he’s a boy.
Nobody knows who this mysterious heir is since Calia mentioned only having a daughter and everything else is hypothesizing who’s kid this is.