It’s only “unknown” from a scientific skepticism perspective. The scientist is saying, “all the evidence that we can gather points with ubiquity towards this being plastic, but there is some possibility that there is an as yet undiscovered particle that is otherwise elusive to detection.” That just being a good scientist. The mark of a scientist that you should not trust is the one who says, “we know this to be true with absolute certainty” instead of “all of the evidence I’ve gathered leads me to personally believe with absolutely certainty, but there could be something else going on I haven’t considered.”