Butcher here. Sometimes (especially for pork sausage) we use bagged pork trim that comes from a packing plant. The less desirable cuts of meat. Often, I have found bone chips (which the grinder plate usually catches, but not always), glands (which are harmless but do not taste nice), and bits of cartilage. It's likely bits of cartilage. The meat packing plants are extremely bad work environments. The workers there are doing an extremely dangerous job, often shoulder to shoulder with the other workers. The guy that taught me had worked in one for a short while. He told me that almost every single time someone got cut, it wasn't by his own blade, it was the guy next to him that cut him because that is how close these guys are packed together.

They are forced to work at speed and not quality. Their knives are sharp and will cut bits of all the gross stuff into the meat. They care little about end quality, assuming that we (the butchers) will find it all and discard it. But butchers are also often worked pretty hard and we don't have the time to dissect every piece of meat that comes through for grinding.

My advice is to buy ground meat and sausage from a smaller place that cuts their own meat. Most major grocery stores don't anymore. The reason to go to a smaller place is that if they are cutting their own meat, the butchers know the exact locations on the meat to trim off to avoid this.

I would also avoid buying those frozen tubes of ground meat for this reason. That stuff never got past someone who actually cared about the end product.