Barney isn't the one banging on the door.

It is Barney, just before he had an unfortunate accident considering a pool of liquid coming out a garbage disposal tank near the canteen.
He had to get a really expensive face replacement, that’s why he still owes Gordon a beer as the operation cost Barney all of his money.

Well, I did not expect that. Bravo sir.

Real explanation, they made it as close to Barney as they could without making a custom model that nobody would notice without noclip.

Where’s the source for the Barney’s backstory?

It’s Barney’s twin brother with a mustache, Arnie Calhoun. One time, he shaved his mustache and Gordon gave $20 to the wrong Calhoun for beer payments, and now Arnie is $20 richer. Goddamn you, Arnie.

Something something vague canon, something something impossible to tell during normal gameplay, something something who even cares

Yeah someone on reddit explained it to me, my bad.