When Stoick the Vast went to his son's room to wake him up for his big day, he was gone. Stoick thought nothing of it, thinking that his son must be out in the forest, clearing out his head, or perhaps in the forge, working on some crazy invention to kill the beast, so he was not the least surprised or concerned when Hiccup did not show up for breakfast. But when Hiccup did not show up to the ring, Stoick grew angry. His son was missing his chance! His chance to prove himself worthy of being the Haddock heir.
Hiccup did not return that night or the next. On the third day that Hiccup was gone, Stoick named Snotlout his heir to the Haddock throne until Hiccup returned, however long that may be.
A week passed and Stoick sent out word of his son's disappearance to all the neighboring tribes. None had seen him.
Fishlegs took note of everything that happened before Hiccup had disappeared and mentioned casually to Stoick that Hiccup had been going off someplace after dragon training to do something, but they had no idea what.
Then other things became clearer. Dragon scales in the cove, missing money from the Haddock's house, and all of Hiccup's belongings were gone along with 2 baskets. No freshly cut tree stumps (as if they expected Hiccup of being capable of cutting down a tree with his runty arms) so he couldn't have made a boat unless he stole one from the harbor and no one them were missing expect for a rowboat that everyone believed that Hiccup had taken until the twins returned it 2 years later saying that they had hid it. But no one knew where Hiccup went expect for the gods and the boy himself.