It was a dreary Friday in October 1976 when 16-year-old Trenny Gibson arrived at school in Knoxville, Tennessee and found out her horticulture class was going on a hike in the Great Smoky Mountains.

The teenager knew they were going on a field trip, but it was only when they filed onto the bus that their teacher, Wayne Dunlap, informed the 40 students where they were actually going.

Trenny was known as a good student who stayed out of trouble. She was petite at just five-foot-three and 52 kilograms and had little experience with outdoor life, but dreamed of one day studying landscape architecture. She had a keen interest in plants and trees.

On the day of the surprise excursion, she had forgotten a coat. Her brother's best friend, Robert Simpson, was in her class and lent her one.

Watch: What happened to Trenny Gibson? The teen who went on a school trip and never came home. Post continues below.

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What happened to Trenny Gibson? True Crime Conversations

Once at the base of the mountains, Trenny and Robert set off together. He has said they split up because she wanted to head back, but he wanted to "track a bear."

It's an excuse that doesn't sit right with hosts Tim Pilleri and Lance Reenstierna of the American podcast Missing.

"There are definitely bears in the park, and you don't want to meet one," Pilleri told Mamamia's True Crime Conversations.

"I think from our point of view, it's almost an excuse for him to not be accounted for, I would say."

Trenny Gibson went missing aged 16. Image: Laura Riste/Trenny Gibson blog.

A group of students remembered seeing Trenny on the path alone.

They watched her bend down and then turn right off the Forney Ridge Trail, seemingly looking at something off the track.

She was never seen again.

Later, searchers found empty beer cans and cigarettes at the exact spot Trenny was seen ducking off at.

Tracker dogs were brought in to find Trenny's scent, and all of them traced her to the same spot; a nearby road, where all traces of her vanished.

Trenny went missing while on a hike with her horticulture class. Image: Google Images/Kamal Khati.

Mr Dunlap was the only chaperone for the 40 students. Image: Laura Riste/Trenny Gibson blog.

"I think it was presumed that she could have gotten into a car at that spot, willingly or unwillingly," Pilleri said.

Investigators did spend some time thinking Trenny had simply run away, but Pilleri "doesn't think that was a possibility in Trenny's case."

Robert's story.

Following Trenny's disappearance, Robert chose not to talk to the police.

His father was the Assistant District Attorney in Knoxville at the time and counselled his son to stay quiet.

"It does seem like he might have had a little bit of a crush on Trenny," Pilleri said.

"It's speculated that something happened between them (on the hike), whether he made a move on her or said something to her that she didn't like.

"But that is speculated to be one possible reason that she kind of separated from him and walked away on the trail."

Robert Simpson was a year older than Trenny, and friends with her brother. They started the hike together. Image: Laura Riste/Trenny Gibson blog.

Once the search for Trenny was in full swing, Robert allegedly entered the Gibson home and screened media and police calls.

"That's odd," said Reenstierna. "That's super suspicious in my opinion."

Trenny's comb — that she always kept on her — was also found in Robert's car.

"Apparently, he would use it on his own hair," Pilleri said.

Robert also tried to point the finger at another student. However, according to Reenstierna, the principal of the school later attested that the student in question was present in class that day.

Trenny also used to wear a sapphire necklace and ring gifted to her by her mum. They had sentimental value and significance to her and she wore them daily.

After she went missing, another female student was allegedly seen wearing the pieces.

Listen to Trenny Gibson's story on True Crime Conversations. Post continues after podcast.

"This account comes from Robert Simpson. He claimed that at the field trip Trenny was in a washroom and gave the jewellery to one of the students to hold. Unclear why that would have been the case, but later, it was a different student who was wearing the jewellery," Pilleri said.

As Reenstierna pointed out, "it starts to have this feeling like there are many other people who know what happened."

A forgotten case.

Over the decades, Trenny's case has blended into history.

The 16-year-old's disappearance left her family fractured, and her case remains unsolved.

The Gibson family home in Knoxville. Image: Strange Outdoors.

"I think it's pretty unlikely that the case is solved, or even that Trenny is found. I think I would be pretty surprised to hear that she was found in the park at this point, just based on the multiple searches [there] over the years," Pilleri said.

But Pilleri and Reenstierna think it's important to keep telling Trenny's story, because over time parts of the story are being rewritten (like the narrative involving Kelvin).

"She was an innocent 16-year-old girl. This is exactly the kind of person that we should be remembering," Pilleri said.

"I don't think she went missing intentionally. I don't think she did anything wrong here. I think it's a tragic story, and I do think she was likely murdered. So, I think it's important to keep victims, survivors, missing people's names out there — because if we don't, who will?"

Feature image: Laura Riste/Trenny Gibson blog.