I'm now completely convinced this is it. It checks all the boxes for me. It's something I never would have guessed. It's something that I could reasonably see as working. It's an absolutely massive industry with a constant need for more product. And it's something that if it were happening, I would presume the industry would absolutely not want people to know about. People buying low fat dairy products or milk alternatives often do it because they see it as being healthier. Would they still want to buy it if they knew there was glitter (plastic or otherwise) in it? I doubt it. I can see a bunch of people rioting because they've been drinking plastic soy milk or whatever.