[dragonwriter]

> The origin of the phrase is in the classical detective story trope.

Which, if I'm not mistaken, itself doesn't derive from household staff taking the fall for their employers, but from them being omnipresent but, by strong social custom, ignored.

[coldtea]

Yeah, I'm fairly sure that's the case. So while checking the motives of the high society suspects, nobody would suspect the lowly butler -- which, in another trope, had some dark past related to the family he was employeed at...