Warning: Spoilers ahead "Stranger Things" season two.

"Stranger Things" star David Harbour revealed the meaning behind the bracelet Hopper wore for two seasons before giving it over to Eleven.

The bracelet is actually his late daughter's hair tie.

"It becomes a thing that's like a security blanket for him, it's a reminder for him," Harbour told INSIDER. "And you'll notice throughout the series I will occasionally play with it like when I'm talking to Eleven or going through a difficult time."

Sara's death may have a link to Hawkins Lab and Hopper's mysterious past, as Harbour teased based on more hidden details.

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The second season of Netflix's hit drama series "Stranger Things" contained subtle nods to an important mystery concerning Hawkins Chief of Police Jim Hopper, played by David Harbour.

Hopper's first daughter Sara and his mysterious past might be connected to Hawkins Lab, as revealed by the blue hair tie the character wears around his wrist.

"It's something subtle we do, and in fact if you watch the first [scene] of Jim Hopper in season one, he wakes up in the morning and before he even checks his watch he touches that bracelet on his arm," Harbour told INSIDER following the premiere of "Stranger Things" season two. "It's the first thing that he does every morning because he never wants to forget her. Sara and the death of his daughter is his grounding place for reality."

Harbour tweeted about this small costume detail after a fan asked him about Hopper's accessory.

The hair tie first appeared on the opening episode when Hopper is waking up on his couch.

Hopper's first scene on "Stranger Things" prominently showed the bracelet. Netflix

The hair tie next appeared on the season one finale when Hopper had a flashback to his daughter falling mysteriously ill.

"You'll see in the first flashback with Sara that she has her hair up in these little pigtails and she's wearing these blue hair ties," Harbour said.

Hopper and Sara on season one, episode eight: "The Upside Down." Netflix

This was the flashback where Sara appeared to be hyperventilating and Hopper was telling her to take deep breaths.

"And then when you see me with her in the hospital after she's going through chemo, and we've shaved her head, he's wearing it on his right wrist when he's reading to her," Harbour said. "So it's this bracelet that was her hair tie, and he wanted to wear it since she couldn't wear it in her hair anymore — it was a little thing between them."

Hopper and Sara sitting in a hospital bed together. Netflix

"And then it becomes a thing that's like a security blanket for him, it's a reminder for him," Harbour said. "And you'll notice throughout the series I will occasionally play with it like when I'm talking to Eleven or going through a difficult time."

Jumping forward to the second season, years have passed between Sara's inferred death and the present-day events in Hawkins. Hopper was arguing with Eleven when the hair tie came into play once more.

Read more: 13 details you might have missed on the second season of 'Stranger Things'

Hopper fiddling with the hair tie on season two, episode four, "Will the Wise." Netflix

After Hopper and Eleven had a huge, blowout screaming match, he went to her door. Struggling with deciding whether or not to take a soft approach to reconciliation, Hopper toyed with the blue hair tie around his wrist. But he steeled his expression and went the tough-love route, telling Eleven through the door that she needed to clean up the mess she had made the night before.

"I'll fiddle with it, almost like you would with a wedding ring when you're going through a divorce but you're still wearing it," Harbour said. "It's a constant reminder to him of his struggles and his guilt around not being ableto save his child."

But by the second season finale, Hopper and Eleven had reunited and made up.

He took her to the Snow Ball middle school dance, and it was there the hair tie resurfaced again: Eleven was wearing it around her wrist as she danced with Mike.

Mike and Eleven dancing on season two, episode nine, "The Gate." Netflix

"At the end of the series you'll see that Jim is not wearing it anymore, but I don't want to to get too into that because that's something we may talk about sometime in season three," Harbour said. "It's something we'll delve more into in terms of Jim's daughter and him confronting this Eleven relationship and what that means ... there's more to be revealed there."

The death of Hopper's daughter might be connected to Hawkins Lab

In addition to this connection between Sara, Hopper, and Eleven, we noted in our initial season two details breakdown that Sara's hospital room appeared to be in Hawkins Lab. The staircase showing Hopper crying after Sara's death was the same staircase seen on episode nine of "Stranger Things 2."

Here's the staircase as seen on the first season, in Hopper's flashback:

Hopper in the staircase in a flashback from season one. Netflix

And here's the Hawkins Lab staircase where Dr. Owens was found by Hopper and Eleven:

The same handrails can be seen in this shot of Dr. Owens from season two. Netflix

We asked Harbour if he could tell us more about this connection between Sara's death in a hospital room and Hawkins Lab.

"Hmmmm [laughing] I can not," Harbour said. "But yes, it is the same location. So that's interesting. Or it's a similar location. But there's actually another thing you'll notice which is that there's a stuffed animal lion that appears in Eleven's room when he breaks into the lab and finds that room, and it also appears in Castle Byers, and it also appears on Sara's lap."

We had pointed this out stuffed animal pattern after season one, though Harbour's confirmation of the link between this animals is a huge step towards figuring out what might be going on.

The three stuffed animals David Harbour described. Netflix

"So yes — there is a connection there perhaps that we may go into further down the line," Harbour said. "Again these are little things that highlight mysteries that I don't want to give too much away from but I don't think anything is done randomly in this show. So the fact that you're picking up on these things is something that will pay off in some way."

The third season of Netflix's "Stranger Things" premieres Thursday. Fans will have to see if any more secrets around Hopper's backstory are revealed with the new episodes as Hawkins faces yet another threat.