Meaning of exodus scene in S1E8 of Stranger Things?

Question

I am looking for an explanation of a mysterious scene early in Ep 8 (Season 1) of Stranger Things.

At about 7min in there is a large movement of people out of Hawkins National Laboratory, with this dialogue between Dr Brenner and Connie Frazier:

Connie: I'm telling you, this is a mistake.
Brenner: It's gone. Isn't that what you wanted?
Connie: And if they find the boy?
Brenner: That's not going to happen.

I get that Brenner did not think Joyce and Hopper would be able to find Will Byers in the Upside Down, because no one Brenner sent in there survived for long.

Were all these people going after El, or were they evacuating the lab and if so why? Why did Connie think it's a mistake? What did Brenner mean when he told Connie "it's gone"? Assuming he meant the demogorgon, why would he think so, or if he was lying to Connie, why?

Answer 1

Were all these people going after El, or were they evacuating the lab and if so why?

The people we see getting into vehicles were going after El. We see both Brenner and Connie at the school later, and the soldiers would've been going to secure the perimeter and search the school.

However, the lab would also be getting prepped for evacuation, as the experiment was effectively over at this point. Their cover was blown.

What did Brenner mean when he told Connie "it's gone"? Assuming he meant the demogorgon, why would he think so, or if he was lying to Connie, why?

By "it," Brenner meant "the problem." He wasn't lying to her, but he was assuming Joyce and Hopper would die in the Upside Down and the problem of all these locals digging into Will Byers' disappearance would be resolved. This, of course, turned out not to be the case.

Why did Connie think it's a mistake?

Connie thought it was a mistake to let Joyce and Hopper go into the Upside Down after Will. That it was a mistake to admit to these civilians what was really going on, and that it was a mistake to assume this course of action would actually solve the problem. Firstly because they might survive the experience (they did) and secondly because they might find and recover Will (again, they did.) If Will Byers turned up alive after the whole fake body/funeral song and dance, it would be that much harder to cover up what they had really been doing there.