Is ‘Not a peep’ an idiom, or just plain statement?

I found a phrase ‘not a peep,’ in the Washington Post’s article (September 16) written by Eugene Robinson, which was captioned “Where are the compassionate conservatives?” In the article, Robinson describes the scene of Republican Presidential candidate, Ron Paul’s answering the moderator, Wolf Blitzer’s question at the Tea party debate held on September 12th:

“Wolf Blitzer asked Ron Paul a hypothetical question about a young man who elects not to purchase health insurance. The man has a medical crisis, goes into a coma and needs expensive care. “Who pays?” Blitzer asked.

“That’s what freedom is all about, taking your own risks. This whole idea that you have to prepare and take care of everybody. . . .”

Blitzer interrupted: “But Congressman, are you saying that society should just let him die?”

There were enthusiastic shouts of “Yeah!” from the crowd. “You’d think one of the other candidates might jump in with a word about Christian kindness. Not a peep.”

What does “Not a peep” mean here? Does it mean “No sound, no voice”, or “No answer”? Does it mean the Congressman was unable to talk back even a single word, or other candidates zipperd their mouth, or audience who shouted 'Yes' in accord suddenly fell into silence?

I checked online dictionaries for the words. None of Cambridge, Merriam-Webster, or Urban dictionary has entry of “Not a peep.” I don't know why. Isn’t this an idiom, or just an abridgement of ‘There was no peep.”?

Merriam-Webster has the following definition for the noun peep:

a slight utterance especially of complaint or protest .

That's the definition referenced in the phrase "not a peep."

The basic sense is 'not a sound', but in practice it often means 'not a word on a particular topic', as it does here: here it means that none of the other candidates made the slightest objection to the idea that society should simply let the young man die. Similarly, a parent might say to a child ‘And when I tell you to do your homework, I don’t want to hear a peep out of you!’, meaning that the child is not to make any vocal objection. On the other hand, if the parent says ‘Go to your room, and don’t let me hear a peep out of you for the next hour’, it means that the child is to be quiet in his or her room for the next hour.

The idiomatic phrase "not a peep" almost certainly originates from the sounds that baby birds (and in particular, baby chicks) make as nestlings. Consider for example, this instance from Henrietta Peck, "Fowl Literature," in the [Chicago Illinois] Prairie Farmer (July 15, 1871):

Twenty-one days at last dragged its weary length to an end, and in a state of great expectancy, I called upon Bessie and Josepha to count the little valuables which they had been able to produce from my choice eggs Not a peep to be heard about the nests. The eggs were too valuable to be examined as other eggs would have been, so I waited another day, and another, and another. Then I found one solitary chicken under each hen—both buff Cochins.

Examples of idiomatic use of "not a peep" go back further still. From an untitled item in the Joliet [Illinois] Signal (November 7, 1848) reprinted from the [Joliet] True Democrat:

The meeting adjourned and gave 3 hearty cheers, that made the Court house ring again, for old Zach. The barnburners then raised a faint cheer for Van Buren, but not a peep could be raised for Cass.

From "Political News" in the Cleveland [Ohio] Morning Leader (November 21, 1860), quoting a reporter for the New York Evening Post who is writing from Washington, D.C.:

Many of the office-holders here are reckoning upon a continuance in office, and especially those who have friends in democratic Senators. They claim that Mr. Lincoln's cabinet officer cannot be confirmed without the consent of their senatorial friends, and the price of that consent will be the retention of certain democratic clerks in office. A good deal of speculation and bargaining—always on paper of—this kind is going on at the Departments, and not a peep of treason can be heard in any of them. Even the South Carolina clerks say they will remain in office just to oblige Mr. Lincoln. They are not afraid of South Carolina terrorism in Washington.

And from "The News" in the Cleveland [Ohio] Daily Leader (November 29, 1865):

Thirty-two women on a railroad train from Canada to the United States, the other day, had babies. A shrewd government detective on board observed that they were the quietest little bundles of juvenile humanity he ever saw, as not a peep was heard from them on the journey. This led him to investigate the baby question, which resulted in the discovery that what were to be little dears, were tin cases containing from five to ten gallons of whisky each. An ingenious method of smuggling, truly.

I've been familiar with the expression "not a peep" from a fairly young age, having run afoul of it as a third-grader. Our teacher had lined us up in a double row prior to walking to the school library and had appointed me as a "hall monitor" to help keep order in the ranks. She then told the class, "We must walk quietly because students in other classrooms are still at work. Remember: not a peep from anyone." Instantly it struck me that it would be immensely droll to say "Peep!" in as deep a voice as is possible for a nine-year-old to manage. So I did—and was instantly stripped of my special rank and relegated to the back of the line, where I could be kept under special observation during the brief march to the library. The lesson I learned: in comedy, timing is everything.

I am not an expert on the English language (a B student many years ago), but as I sit in my chair with 10 baby chicks near, I find it easy to think that this is a chicken quote because someone is always peeping. Which begs the conclusion that it is a rare occurrence when not a peep was heard. I would say it is a form of plain speaking by an idiom.