But it's not an 'elephant in the room' either - the elephant has already emerged but is being ignored. The 'n****r in the woodpile' has yet to be revealed.
There are no other English phrases I can think of that describe an obscure factor or a concealed motive quite as well as "n****r in the woodpile", so for a 60 year-old woman to blurt it out is wrong but forgiveable if used in a specific context.
However, there's nothing hidden or concealed...