CUPERTINO — A Cupertino teen who went missing Monday morning and returned home late Thursday night told authorities he ran away from home after making “a spontaneous decision,” according to the Santa Clara County Sheriff’s Office.

Connor Sullivan, 17, told sheriff’s deputies he spent the first day walking trails in the Fremont Older Open Space Preserve, located near Monta Vista High School, where he attends. For most of the time he hid and slept under the bleachers at Monta Vista and ate fruit on various trees on and around campus, according to sheriff’s Sgt, James Jensen.

“No one knew he was hiding,” Jensen said. “No one saw him. No one was helping him.”

Thursday after sunset, Connor emerged from the school bleachers and walked home, surprising family and friends who had brought over pizza.

On Friday, his family issued a letter of thanks to law enforcement and the community, and an apology. “Connor has expressed the deepest sorrow and regret for his actions,” the letter signed by Jeff, Jill and Connor Sullivan read. “Over the next few weeks we will work on the how and especially the why of his disappearance.”

The teen told sheriff’s deputies he was stressed about school but it was not the reason for running away. Jensen did not disclose if Connor gave authorities a more specific reason for his actions.

After talking to the teen, the sheriff’s office doesn’t believe there was anything suspicious about his disappearance, according to Jensen.

Connor was reported missing Monday after he did not show up to track practice at Monta Vista.

After Connor went missing, the sheriff’s office activated the Cupertino Emergency Response Team, and within hours hundreds of volunteers scoured Cupertino looking for the teen. Search parties also checked local malls and hospitals.

“This type of response happened because it was so out of character,” Jensen said. “He never ran away before. He goes to school, goes to practice and comes right home.”

Contact Mark Gomez at 408-920-5869. Follow him at Twitter.com/markmgomez.