Check this dude's thread out. He seems to have the same issue.
https://forums.macrumors.com/threads/1489655/
At this point I'd take it in.
I noticed that in my camera too and taking my in today to get it replaced along with slew other issues with the phone.
Yikes :/ lots of them then
I went back to some old pics..and didnt notice them and then I was taking a pic of a document for scanning using genius scan and was like wth is that..Seems like i crept up in the lens somehow.mmmm.
Yeah that's exactly what happened with me too. First noticed it when I took a picture of a receipt.