The last known sighting of Gannon was recorded by neighbour Roderrick Drayton's CCTV camera on January 27.
He says it shows stepmom Tecia Stauch backing the pickup into her drive, and Gannon getting into the back seat at 10.13am.
Tecia returned home at 2.19pm, according to the security video, but Gannon was nowhere to be seen.
She told cops he left home between 3.15pm and 4pm to walk to a friend's house but did not say which friend.
Footage from that time was not available, but the sheriff's office said they had no video evidence of the boy leaving the house at the time his stepmom said he left.
The video emerged nine days after Gannon went missing as searching the suburban town south of Denver.
Roderrick said he trawled through his security video hoping it might help the search.
He : "I couldn't sleep so I just stayed up and just kinda kept looking through."
He also told ABC News: "I just kept searching and I saw where she came back.
"And he didn’t get out of the vehicle. So she came back by herself about four hours later."
It was claimed Gannon looked "sluggish" in the footage, as if he had been "drugged".
Rodderick said he showed the video to Al, and added: "He just broke down crying and said, 'she lied, she lied about the time. He didn't go to a friend's house'."
The El Paso County Sheriff's Office said it has a copy of the footage, but urged the public not to draw conclusions.
Sgt Deborah Mynatt said: "Please understand, we wanted that kept ... because it is a big part of our investigation.
"And to share it immediately, we hope will not be detrimental, we hope it will not jeopardize the investigation."
Gannon was initially treated as a runaway, but the investigation was upgraded to an "endangered missing child case", .
Police said that takes account of how long he's been missing, the low overnight temperatures and his need for medication.
Stepmother Tecia has claimed she was treated as a suspect and denied a lawyer when investigators grilled her last week.
She also claimed her 17-year-old daughter was briefly handcuffed when detectives pulled their car over on Thursday.
She deputies had guns drawn and told her they were going to shoot her without identifying themselves as law enforcement officers.
Tecia claimed they also refused her water and bathroom breaks, and wouldn't let her have a lawyer as they asked her about the disappearance.
She said: "During that time, some of those things made me feel uncomfortable the way they were saying things.
"So I immediately stopped and felt like an attorney would help me with some of the vocabulary and things like that that I needed help with and understanding some of the things they were asking."
Tecia also claims she has had more than 20 death threats online.
She insisted she would never harm her stepson and had taken care of him for the last two years.
Her husband Al Stauch, Gannon's dad, was reportedly out of town at the time.
He said his son has never run away before and was not like him to walk off.
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Last week Gannon's birth mother Landen Hiott made an emotional appeal to help find him at a press conference.
She said: "I'm so thankful for all the outpouring of help this case has brought.
"My son is a very loving kid. He wouldn't want harm on anybody at all and it's so hard to think, 'Why is this happening to him?'
"I have no clue. My kid deserves to come home. My kid has a purpose, my kid has a life and is important to me and is important to everybody that is standing in this room.
"Bring my baby home, I'm begging you."
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