I think Harumin has special feelings for Matsuri that she herself is unaware of, and that's being hinted at for a while now.
Harumin is perceptive with people's feelings but doesn't want to get involved, even if it's about Yuzu who is her best friend. But Harumin seems compelled to help when Matsuri shows stronger feelings. I recall during the Summer camp arc when Matsuri was going hysterical over the discovery that Mei would get engaged, that Harumin hugged her tight and dragged her away.
Now she is the first to notice Matsuri is gone and will likely confront her over it. Gay!