Well, if there was any doubt of it, with this chapter now it's clear that intelligence is not Momo's strength. I'll give her a bit of slack given that the “Do Not Open” sign was down on the floor, but still. Guess that common sense is not actually common at all, neither IRL nor in most fiction works.
Even if the others know that she's on a drugstore, it's going to take them a while to get there to save Momo... I don't think that the author is gonna go that far (not yet at least), but given Saya's nightmare, I'll have a little faith.