There’s a lot of outsourcing on these big games these days, so likely, some random contractor knowingly took the images and made a texture out of it, passing the work off as their own. And Bungie didn’t really have a way to verify if what they paid for for was original or not.

It was only identified because the original artist recognized their own work.

I’m not sure how a company can actually proactively prevent this kind of thing. Even doing all the work in house runs the risk of a rogue employee lying.