The mask in Shinobu Kocho's hands was, of course, one Kanzaki Kumo had commissioned from that shrine beforehand.
Although Kanzaki Kumo had not learned the exact location of the shrine's stall ahead of time, so as not to spoil the fun of attending the festival, as long as it had a stall there, they were bound to come across it sooner or later.
When Kanzaki Kumo and Shinobu Kocho split up to look for masks they liked, he took advantage of a moment when she was not paying attention to give the shrine maiden selling masks their agreed-upon signal. Then he picked up a mask himself, ready to give Shinobu Kocho a scare.
While enjoying a little playful interaction as a couple, he also provided cover for the shrine maiden, letting her secretly place the custom-made mask near where Shinobu Kocho stood.
Thanks to Shinobu Kocho's "cooperation," the plan went very smoothly.
So smoothly that Shinobu Kocho immediately figured out the whole story behind the mask.
After all, it was too much of a coincidence not to make someone wonder.
Shinobu Kocho: "Could it be that I happened to catch that shrine maiden's little maneuver?"
Kanzaki Kumo: "Huh?"
It was only a little surprise during her birthday celebration. Once the mask was in Shinobu Kocho's hands, whether she figured it out no longer mattered.
If anything, having Shinobu Kocho work it all out and then tacitly agree with Kanzaki Kumo to chalk it up to "luck" was even better than her not figuring it out.
Was Kanzaki Kumo supposed to explain everything afterward and spoil the surprise? Or keep it bottled up inside and act as though nothing had happened?
Compared with those two possibilities, things were naturally much better as they were.
And so Kanzaki Kumo continued strolling through