I have not seen this specifically, but I have 2 thoughts.
This could be layover from manufacture. I have seen that sort of thing before where something in the process did not get removed properly before it was fully assembled straight of the line. I have seen stuck bits of plastic like this on iPhones and MacBooks more than once.
The other thing that springs to mind is remants of the the adhesive liner. When you put new display adhesive in, there is a plastic lining that the adhesive is stuck to and you peel it off as part of the repair. It's possible it just fell in when they were replacing it.
Otherwise I'm at a loss.