I wonder- this could just be Miko being mindful of Hinako, but I don't buy it. If she had just told Shiori not to trouble her on that day, that would have been one thing. But to go as far as to indirectly tell her where she would be able to find her, come Monday....
Could it be possible that she actually wanted, on some level, for Shiori to make sure that Hinako wasn't alone that day?
After all, Miko couldn't do it herself, her relationship with Hinako goes back so far, and there are so many unspoken rules and promises between the two, if she were to visit her on the anniversary of her family's death, that could actually severely damage their relationship.
Shiori, on the other hand, has proven often enough that she does not care much about those kinds of rules, to the point that it feels less personally offensive if she simply breaks them. She can actually be there for Hinako, and I have a feeling that maybe, knowingly or not, that's what Miko was going for here, because I think as much as she dislikes Shiori, she seems to also have realised that she only wants the best for Hinako.