The grey market doesn't mean stolen - it just means unauthorized distributors. Companies building a product will end up with excess parts -- either they bought too many to satisfy minimum purchases, or changed the design, or cancelled the product. They sell these parts to brokers, who then resell them. The way to avoid this is to limit sole-source suppliers and make sure there are backup plans (parts that fit with same footprint, etc.)

>What does the grey market for components even look like? I can't think that anybody is bothering to steal individual components, so are we talking possibly-counterfeit parts?

So somewhat related, check out Strange Parts on YouTube. His past couple of few videos involve him building a Samsung Galaxy from components in Shenzhen and then having it x-rayed by a place that (as one of their functions) identifies counterfeit parts.

In the video with the x-ray they talk about reclaimed and counterfeit parts and you can see differences between what was supposed to be a real camera module and an actual real camera module in one of the x-rays.

https://www.youtube.com/channel/UCO8DQrSp5yEP937qNqTooOw