Here's an interesting passage from John 20:3-8hughfarey wrote: Maybe the reason there is no mention of the Shroud in the Gospels is that the image did not miraculously appear until much later
In this passage, the Apostle John doesn't immediately enter the tomb when he reaches it. From outside the tomb he looks in and sees Jesus burial cloths.3 So Peter and the other disciple started for the tomb. 4 Both were running, but the other disciple outran Peter and reached the tomb first. 5 He bent over and looked in at the strips of linen lying there but did not go in. 6 Then Simon Peter came along behind him and went straight into the tomb. He saw the strips of linen lying there, 7 as well as the cloth that had been wrapped around Jesus’ head. The cloth was still lying in its place, separate from the linen. 8 Finally the other disciple, who had reached the tomb first, also went inside. He saw and believed.
When Peter reaches the tomb he immediately enters it and after Peter enters the tomb then John decides to go in as well. It is only after John enters the tomb that the narrative says "He saw and believed"
Evidently there was something that John saw only after entering the tomb that was not visible to him when he saw the burial cloths from outside the tomb.
What did John see that caused him to believe before he actually saw the risen Jesus?
The only thing that the text mentions is Jesus burial cloths.
The implication of the text is that John saw something after entering the tomb, that was not apparent to him when he saw the empty burial cloths from outside the tomb.