The gospels aren't generally written from an Omniscient Narrator perspective, though. That and the temptation in the desert scene are the only stories that take place out of sight from the disciples.
I think there's a few more than that.
Matthew 1. We're told that Mary was "found to be with child by the Holy Spirit." We read about the secret plans of Joseph to deal with Mary's pregnancy. Then we're told that an angel visited Joseph in a dream. Then we're told that Joseph married Mary, but didn't have sex with her until for many months, until Jesus was born.
Which of the disciples or gospel writers observed all that?
Matthew 2. Herod "secretly" asked the visiting Maji where the star they had been "following" was pointing. After the Maji visit Jesus, they are warned by God "in a dream" not to go back to Herod, and instead exit stage right never to be heard from again. How did anyone know what their dream was about?
Joseph has another dream about fleeing to Egypt, and a third dream about returning when Herod died. Then he has a fourth dream about moving to Nazareth.
Matthew 3, Luke 3. John the Baptist baptizes Jesus, and a voice from heaven declares Jesus a beloved Son. This might have been in front of a public gathering, but none of the disciples where there, were they? They certainly didn't mention anything about it later.
Matthew 4, Luke 4. The temptation of Jesus. Lot's of one-on-one dialogue between Jesus and a spiritual being, but this has already been mentioned as an exception.
Jesus finds out that John the Baptist has been arrested, so goes to Capernaum. This was before he called any disciples.
Matthew 14, Mark 6. The daughter of Herodias dances before Herod, and he lets her name a reward. "Prompted by her mother" she asks for the head of John the Baptist. Which disciple was present at this royal gathering, and which of them knew about this secret conversation between a mother and her daughter?
Matthew 26, Mark 14, John 11. Chief Priests and Caiaphas plot to capture Jesus and kill him, "but not during the festival." Which of the disciples knew the details of this secret plot but chose not to say anything about it to Jesus?
Judas alone betrays Jesus and is paid thirty pieces of silver. Did he disclose this sum to anyone? He also arranged ahead of time the secret signal of kissing Jesus.
(Also Luke 22) The Garden of Gethsename. Already mentioned as a lengthy passage where Jesus has a long conversation alone with God.
(Also Luke 22) Jesus is arrested, and the disciples flee. Then we learn the details of his trial with Caiaphas in great detail. Peter was there, but outside in the courtyard. I doubt he was jotting the record down in a notebook.
Matthew 27. We learn that Judas, who certainly wasn't hanging around with the other disciples, feels remorse, returns the betrayal money, converses with the priests, and hangs himself. The priests consult with each other what to do with the money. Meanwhile, the disciples are all in hiding.
(Also Luke 23, John 18) Jesus has a conversation with Pilate. Were any of the disciples present as character witnesses?
Pilate's wife tells him that she's had a dream about Jesus. Which disciple was privy to the dreams of Pilate's wife?
When Jesus dies, the veil of the temple (accessible only to special priests) rips "from top to bottom." That's a very interesting detail which none of the disciples were in a position to observe.
Chief priests and Pharisees convince Pilate to post a guard on Jesus' tomb. Somehow the scattered, grievous disciples know about the details of this conversation.
Matthew 28: The chief priests bribe the soldiers guarding the tomb. Was this done out in public?
Mark 1: More details about Jesus' baptism, and the wilderness temptation, and angels "ministering" to Jesus, all before he called a single disciple.
Mark 15, Luke 23: We learn about private conversations between Joseph of Arimathea, Pilate, and a centurion.
Mark 16: Mary, Mary, and Salome, find an empty grave, are told by a boy that Jesus will see them later in Galilee, then the ladies flee and tell no one anything.
Luke 1: Zacharias has a conversation with an angel, then is struck mute, so whom did he tell the story?
Luke 2: Some random shepherds are told about Jesus' birth. Which disciple did they tell thirty years later?
Mary observes wonders about the child Jesus "and treasured them in her heart."
Luke 20: Scribes and chief priests send spies to entrap Jesus in blasphemy, but when they can't, the priests "remain silent", meaning they didn't chat about that with any of the disciples.
Luke 22: "Satan entered Judas" probably not a public affair.
Luke 23: Were the conversations between Jesus and the thieves loud enough for bystanders to hear?
John 1: Before Jesus or his disciples enters the scene, John the Baptist converses with Pharisees. Who recorded their dialogue?
John 3: Nicodemus meets Jesus alone at night to discuss theology.
John 4: Jesus has a long conversation alone with a woman from Samaria.
John 19: Soldiers consult amongst themselves what to do with Jesus' cloak. Surely they didn't include a Jewish bystander in their negotiations.
John 20: After the disciples leave the empty tomb, Jesus has a one-on-one conversation with Mary.
Throughout the gospels, Jesus has a conversation with someone, and there's no mention if there's anyone else around. Maybe there were one or two--or all twelve--disciples nearby, hanging on every word, committing every scrap of dialogue to memory so that they can repeat it all thirty to fifty years later to the gospel writers (assuming they lived that long.)
But then there are other passages where Jesus commands his disciples to go preach 'two by two' and THEN he converses with someone, implying that no one else was privy to the conversation. There's just really no way to know.
I admit, that if only one or two passages were like this, then the claim that there aren't all that many passages in the Gospels with the Omniscient Narrator POV might be valid. But there aren't just one or two. There are many, which was quite common for the time, and which makes the question about who said what to whom in 1st-century Palestine very difficult to ascertain.
So difficult, in fact, that skepticism about what happened two thousand years ago in a non-literate era is the prudent choice.