Wow, good eye. I didn't even see that. If there is something in there you would think the vet would have seen it, but it definitely looks like something stuck in there with infected tissue surrounding it. I know what I would do in this scenario but I have medical people in my house. All of whatever that is needs to come out and the wound needs to be flushed.
Could be that sliver is just an optical illusion in the picture though.