... The human race.
Between the black market in alien tech, government jurisdiction territoriality, conspiracies and more, humanity gets a rather worse rap in Torchwood than it does in Doctor Who. Being an Earthbound show, more human nastiness is quite natural. There’s certainly room for an episode where the threat is entirely mundane.
Not much to say about this, of course, except that it can be tricky not to make it feel like a cheat or a letdown when the players are here to hunt monsters. Perhaps some hint of the uncanny can be included - like a serial killer being caught trying to escape because a door inexplicably won’t open for him, perhaps.
Example: The Simple Art Of Murder
The team are called in to investigate a murder on an Air Force base, which despite the victim having once chased a UFO, is actually for a mundane human reason...
... even though there have been UFO reports near the base for months, and the PCs spot a hangar under guard, which the base commander dismisses as “nothing to do with this” and changes the subject.