Board Thread:Metal Gear Solid V/@comment-6747658-20150826194634/@comment-156.12.209.51-20150829160530

Another thing pointing out the 3rd individual at the end of Ground Zeroes. Kojima answered in an interview that that was Kojima himself looking in at the characters he's created. And with Big Boss not having the shrapnel at the end of Ground Zeroes, it may not be completely visible but you see broken skin around where the horn supposedly comes out of.

i.imgur.com/xW5gxsf.jpg

It could be passed off as a random bruise, but still it's too coincidental. After those years in the coma, the shrapnel slowly pushed it's way out.