The eponyms page mentions, "judas hole: a one-way peephole in a door."

I'd never heard of this -- until yesterday. Then, reading a trashy novel, I find a scene where the detective knocks on an apartment door; the occupant is distrustful, and he (the detective) says, "OK, I'll hold up my identification, and you can see it through the judas hole."

Was the author just showing off a bit? Or is this a phrase that others have heard, or used?

I've never heard of it....though I am much more interested in what trashy novel you were reading! Hey, we've had threads on fiction, nonfiction, poetry; maybe we need on on trashy novels!