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.

Founded in 2004, became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.