I wonder if this is a reference to that moment Reimu watched Marisa sleep at the end of Forbidden Scrollery. Marisa had been attacked unexpectedly, so after they bandaged her wounds, Reimu sat there and watched her sleep until nightfall, petting her head, with this same worried look on her face.

I like to think Aunn knows exactly how they feel about each other. I like to think she knows a lot of secrets that she will never tell.