Any situation,activity or behaviour that is exposed when wished not to be.---from the old days when washing machines did have ringers and getting you tit caught in it was a possibility!
She thought she would cheet on the test, but she got her tit caught in the ringer!
Similar to a Bellwether, a John Ringer refers to a leading indicator of some sort of change, similar to "canary in a coal mine". Unknown origin, has been used as a common phrase in corporate conference call settings at least going back to the late 90's.
As you can see here on this chart, the strong sales numbers act as a great John Ringer indicating healthy growth.