PHILADELPHIA — There are at least 15,000 members of the media in Philadelphia right now, along with virtually every progressive movement leader and Democratic elected official in the country.

Yet no one seemed aware that hundreds of Bernie Sanders delegates planned to walk out of the Democratic National Convention in unison Tuesday, at the precise moment party unity was supposed to reach a crescendo.

The walkout, which came as Sanders personally moved to nominate Hillary Clinton by acclimation, left hundreds of seats on the convention floor empty. It came after protests from Sanders delegates seemed to be fizzling out, thanks to personal pleas from Sanders and Elizabeth Warren, among others.

Related: Sanders to 'Bernie or Bust' Movement: I'm Not With You

The several hundred delegates stormed out of the convention hall before quietly sitting down and occupying the press tent, aware of how to maximize their leverage. Police surrounded the tent, and refused to let anyone in or out until the situation was resolved.

That surprise was by design, but also a product of the fact that the walkout came together in only the final minutes, according to interviews with more than a dozen organizers. Word spread through various delegations by word of mouth and text message like a wave during the roll call vote of the states.

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"We had to set up the element of surprise," said Mike Grom, an Ohio delegate. Indeed, most of those who walked out refused to discuss details of it's planning.

Paranoid about being discovered, walkout organizers kept more prominent groups at bay, like the Bernie Delegates Network, which has been holding press conferences every day on potential protests by delegates.

“We tried to keep it very hush hush,” said an Ohio delegate named Puja, who refused to give her last name.

It came together organically they said, percolating as Sanders delegates felt each other out to see which ones would be interested in protest.

Eventually, they organized themselves in several secret Slack channels and Facebook groups, the largest of which had over 1,250 delegate members.

Information was kept close to the vest, with word of the walkout only going out to many delegates by text message or word of mouth after they were sitting in the arena.

“I didn’t know it would happen when the roll call voting began,” said Dennis Slotnick, an Ohio delegate who was one of several dozen that quietly occupied the press tent just outside the arena.

In the days leading up the roll call vote, delegates discussed various ideas in the secret groups. Some suggested more confrontational tactics, like the ones used at the Nevada Democratic state convention. But those were rejected.

“It was a bit of throwing spaghetti at the wall and seeing what sticks,” said Washington Richard May, one of the organizers, who said more actions may come.

The good news for Democrats is that they likely have never heard of anyone involved in the walkout. There were zero prominent union leaders, elected officials and activists. The man negotiating with police on behalf of the group, Jeffrey Eide, and the closest thing the delegates had to a leader, runs a bicycle advocacy group in Fargo, North Dakota.

Related: Democrats Pull Back From Brink on First Night of Convention

The bad news for Democrats is that they likely have never heard of anyone involved in the walkout. There is no one for party leaders to work with, since the group is so decentralized, and even Sanders clearly couldn’t stop them.

“This has nothing to do with Bernie, this is our revolution,” said Victor Caula, a delegate from Iowa.

They sat silently because they said they felt unheard by the party, which many said they could never trust. They said the primary was rigged, pointing to the internal DNC emails leaked to Wikileaks, apparently by hackers tied to the Russian government.

Delegates likely over-represent the most die-hard activists, given the time, effort and money required to be one. They will likely never again have the leverage they did Tuesday, thanks to those 15,000 members of the media.

But they do seem represent a wing of the party that Sanders helped bring in the donors, and that aren’t interested in staying in their seats, no matter what even Sanders says.