A variation on "The butler did it".
To read English detective fiction (mainly from between the wars) is to encounter novel after novel in which middle- or upper-class characters, questioned about a murder in their midst, answer vaguely that it must've been committed by a passing tramp.
Passing tramps were the Golden Age's version 2.0 equivalent of today's "terrorists": an amorphous mass of faceless evil-doers out there, somewhere, ready to commit uncivilized atrocities on a whim. (Version 1.0 were the bomb-throwing Russian "anarchists" of Victorian popular fiction.) The reader soon forms a mental image of English lanes and highways thronged with suspicious hobos, lurking around country houses and villages, bent on breaking and entering, killing at random, then slipping away unnoticed, leaving the police famously "baffled".
Why a vagabond would want to knock off complete strangers in (for instance) country house libraries, never crosses the survivors' minds. Perhaps it's a foreshadowing of fiction 70 years on, in which folks don't need reasons to kill, they do so simply because they're "sociopaths", "psychopaths" or "serial killers"--these useful labels saving authors the effort of racking their brains to invent plausible motives.
The irony is that in between-the-wars England, many tramps were World War I troops demobbed back into civilian life, only to find themselves with no place to go. Jobless, poor, forced to beg, they trudged across the country looking for work, sleeping outdoors and scrounging for food. As far as I know, only Dorothy L. Sayers' Have His Carcase (1932) touches on this aspect of contemporary society.
The ludicrousness of "a tramp did it" is made clear in Ngaio Marsh's Artists in Crime (1938). Here, the scene is an artists' studio. A slinky model poses in a prone position on a wooden dais over which a diaphanous swathe of material is draped. One day the model takes up her pose, only to find, fatally, that the diaphanous material conceals a dagger stuck upwards through the dais' planks. What sick mind could've perpetrated such a fiendish act? Why (according to one of the art students), that of a loitering vagrant, who else. It would've been child's play for such a low-life to pause outside the studio window on a previous day, take in the scene and unhesitatingly contemplate murder, and return at night to hammer the deadly dagger between the dais' planks.
Of course, in almost every novel, the real culprit is one of the charming, good-looking, laughing-eyed, conscienceless middle- or upper-class characters. Nevertheless, this didn't affect the popularity of the "passing tramp" mentality in the works of Agatha Christie, Sayers, Marsh and others. One must thank World War II and film noir for its demise, and incidentally, that of the Golden Age of detective fiction.