The source of this phrase is John the Baptist speaking of Jesus.
26 John answered them, saying, “I baptize with water, but there stands One among you whom you do not know.” John 1:26 (NKJV)
The word translated ‘know’ is the Greek verb oida and means to, “Know fully; understand, recognize (The Complete Biblical Library Greek-English Dictionary).
The reality here was that Jesus was walking among the people unnoticed. While He was publicly identified when John baptized Him a short time later, even after this only a few recognized who He really was. For example, long before Peter had his experience (Matt. 16) Nathaniel declared His deity.
49 Nathanael answered and said to Him, “Rabbi, You are the Son of God! You are the King of Israel!” John 1:49 (NKJV)
Like Peter, Nathaniel received this by revelation. In minutes he went from sarcastic doubter (Can anything good come out of Nazareth?) to believer and follower. He now knew the one John had referred to and his heart had shifted.
In a similar manner One is walking among us all the time, walking in our culture, walking in our days, walking in our relationships. As we go about our daily lives He desires to be involved in our daily decisions and actions. Do we understand, recognize and follow Him?