Xbigy Games' Kamran Draeger writes:
"Konami has released a huge chunk from their new Fox Engine demo that was revealed during the Metal Gear 25th Anniversary event and it appears as if Snake could have possibly not been the only familiar face to show up."
"If you listen closely to the English translator in the Trailer at 11:22, he accidentally lets slip who the boy is... 'That area off in the distance, that square area, that's the cage that you saw in the beginning, with the boy, Chico'. "
Props to onyoursistersback for spotting this. The leaked presentation in which this can be heard has been embedded in the article.