The "Stranger Things" Season 4 teaser ends with an eerily familiar sound before it cuts to black. That sound scares Jim Hopper because he knows it comes from a Demogorgon.

The Duffer Brothers released the teaser for the upcoming season as a Valentines' Day treat for the fans. Not only did it excite them for the return of the Netflix series, but it also brought more questions than answers.

One question involves the look on Hopper's (David Harbour) face before the teaser ended. He stared into the distance and he seemed scared. According to Metro UK, before the trailer cut to black, the familiar eery sound of a Demogorgon's wail played in the background.

"My theory is that the Demogorgon will escape the prison and start attacking the Russians. Hopper will see this, which will explain the fear in his face and then him escaping because of the guards focusing on the Demogorgon," one fan wrote.

One viewer even suggested that this event sets up the story for "Stranger Things" Season 4. It would start with Hopper able to track Eleven with the Byers family, as well as the rest of the gang, and he would tell them about the Demogorgon imprisoned by the Russians.

we’re really not in hawkins anymore 😈

— stranger writers (@strangerwriters) February 14, 2020

This could be the same Demogorgon teased at the end-credits scene of Season 3. In that scene, the soldiers were looking for other food (human) to feed the monster from The Upside Down. They decided against the "American," supposedly because he still serves a far more important purpose in "Stranger Things" Season 4.

Then again, Hopper could be looking in the distance for an escape route. He could be charting his path and the thought of escaping scares him because of the many armed soldiers on the premises.

There really is no telling yet as to what is going on in Hopper's mind in this scene from the "Stranger Things" Season 4 teaser. The important thing is, he is alive and could, after all, be the American captive teased in Season 3. Some fans still think that Dr. Martin Brenner is the American especially since Matthew Modine hinted at his return in the Netflix series in a past interview.

David Harbour in Stranger Things
David Harbour reprises Chief Jim Hopper in "Stranger Things" Season 4. Netflix / YouTube