This is a clear example of a "private interpretation" (II Peter 1:19-21). Nowhere in the Old Testament does it say that no one would know where the Messiah was from. In fact, it says just the opposite! Matthew shows that Micah 5:2 names Bethlehem in Judah as the town in which He would be born, and that Isaiah 9:1-2 identifies Galilee as where He would launch His ministry.
Where did the Jews get such an outrageous, unbiblical idea? It was someone's private opinion that over time had become tradition, an accepted "fact." It became a proverb that is just as true as, "If you touch a toad, you'll get warts."
Is it any wonder that the people argued about Him so much? Earlier in John 7, we see some of this:
And there was much murmuring among the people concerning Him. Some said, "He is good"; others said, "No, on the contrary, He deceives the people." (verse 12)
They had no idea what to expect because they were burdened by their traditional yet wrong understanding about the Messiah.
Richard T. Ritenbaugh
No Private Interpretation