they said they had 53 trackers and only 36 "returned usable data" so theoretically some of those unaccounted for 17 could have been recycled... I mean they don't know what happened to them. I think they could have at least highlighted that in the article.

I've heard of this before. Activists put trackers on recyclables and the automatic sorter removes them because they have metal, and the activist gets outraged that the item went to the trash.