It might be a drain for some part of the system. I put one in my hot water system so if I ever need to work on it I can drain it down completely without having to mess around with bowls and rags inside the house. Somewhere inside there will be a valve - possibly a gate valve as they are notoriously useless at sealing properly once they are more than about two days old.
Are your floors suspended or solid? What is inside this bit of wall?
Are your floors suspended or solid? What is inside this bit of wall?