I hadn't seen that detail previously either.
What would powdered fluorescent dye indicate? What could it have come from? And why?
Could be anything, of course, and I don't know how common it would be for Japan, but my first thought is, a New Year festival? Either for kids (super common in my country of origin as New Year is a huge, huge festival).
I haven't noticed it to be a big deal in US, and I don't know about Japan. But thinking of a play for kids, Santa Claus and the Ice Maiden, Santa bringing gifts, a long white beard with glittering powders, not uncommon. Or magic wand? That's usually covered with glitter.
I wonder how big is New Year in Japan? Are festivals for kids before the holiday common?
In WA, we'd take kids to the Nutcracker ballet, it is the fixture here, but i don't know why this thought suddenly came to my mind. Intuitively, popped up, so I am putting it here without any specific tie.
Maybe someone could chip in, how big is the festival in the US (the kids part - would it be important to make a NY tree party for the kids at the base, since we are considering it)? Vs how huge are kids/students pre-NY festivals in Japan?
Vs, is it a huge Korean tradition? (Off to Google).
Another question - packaging is gorgeous in Japan, much better than the gifts themselves, packaging is art. Would glitter be typical for some packaging paper? On the ribbons? Maybe the perp was opening the gifts? Or is gift delivery before the NY common? Did the Miyazawa get gifts delivered?
ETA. Christmas tree ornaments, and this is where you'd get different colors of glitter. Either putting up (less likely, as he won't be walking like this for 10 days), or dismantling. That glitter would be usually thicker than decorative cosmetic.