For those that forgot (since I forgot the specifics and looked back on it because of it): In Chap 19 when she's introduced (well, ch18 technically), Sanya (the one staring at the "beef stew" stone) is implied to somehow be related to Ayume/Ayumu, Tsukuru's sister who died in a crash with her parents, but Tsukuru assumes he's mistaken.
Also in Ch 19, it's shown that when he goes home + finds out they died, he finds a stew on the stove, which Ayumu says in a letter is a feast she's making. It's never directly stated what type of food it is, but from this chapter, I get the feeling that it was beef stew, and that this "beef stew" stone was actually written by Ayumu before she died (in this world, after being car-reincarnated in the original summoning) and got reincarnated as a beast person. So this is probably the gears in Sanya's head starting to turn as she remembers something, or realizes she can read it.