Depending on which estimate you accept, there are about 11 million, or maybe 11.5 million, illegal immigrants currently in the United States. Much of the ongoing debate over comprehensive immigration reform has focused on what to do with them. The Gang of Eight bill passed by the Senate would give them legal status almost immediately, provided they can pass a background check and pay a $1,000 fine over installments in a six-year period. Other proposals would require them to remain “in the shadows” for a few more years while border security is brought up to a higher standard.

The Gang of Eight stressed that illegal immigrants must be quickly legalized because their current anonymity presents a continuing legal, and perhaps a security, problem for the United States. “You have 11 million people here living here right now. We have to find out who they are,” Rubio told Fox News’ Sean Hannity June 19. “We have to run them through a background check. And they have to start paying taxes and they have to pay a fine.”

But one factor Rubio did not mention that night, and in fact none of the Gang of Eight stressed during the debate, is that nobody believes all of the 11 million, or 11.5 million, currently illegal immigrants will apply for legalization if the reform envisioned by the Gang becomes law. Both the Congressional Budget Office and the chief actuary of the Social Security Administration calculated that only about eight million people will be awarded Registered Provisional Immigrant status in the period after immigration reform is passed. That leaves three million, or perhaps three and a half million, illegal immigrants who won’t become part of the new, reformed system. That’s a big number — about one in every four illegal immigrants in the U.S. today. What happens to them?

Will they be deported? Will they stay in the shadows? Will they eventually win an amnesty? Right now, no one seems to know. And what was extraordinary about the Senate Gang of Eight debate was that no one really discussed the issue. And they’re still not talking; Rubio’s spokesman, and also his chief of staff, did not respond to inquiries for this article.

But there are some hints of what is to come. First, there is the question of why currently illegal immigrants would not apply for Registered Provisional Immigrant, or RPI, status. I asked Stephen Goss, the chief actuary of Social Security, and here is his response, via email:

Regarding the 11.5 million “undocumented” individuals residing in the U.S. at the end of 2011, yes we estimate that roughly 8 million will apply for RPI status.  Those who do not fall into a few different groups.  First, a little less than 1 million (about 900,000) are estimated to attain legal permanent resident status directly through other means available under current law without first becoming RPI.   In addition, undocumented residents do not all just stay in the U.S. indefinitely, and some will have already left out country by the time they could apply for RPI.  Also, we simply assume that some will not apply for any of several reasons, including the penalty and application fee, and concerns over qualifying and fear of the outcome should they not qualify.

Goss subscribes to the 11.5 million total figure, so he believes the total number who will not apply for RPI status will be 3.5 million. If 900,000 become legal by other means, and then, say — and this is just a guess — another 600,000 leave the United States, that still leaves two million illegal immigrants in the country who will not become part of the new system.

What happens to them? They are the missing millions of immigration reform.

“There would seem to be three options,” writes Mark Krikorian of the Center for Immigration Studies, which strongly opposes the Gang of Eight bill. “They’d all be deported; they’d leave of their own accord — self-deportation — because of E-Verify (though it wouldn’t be retroactive  under the Gang bill, so it would apply to them only if they were to change jobs); or they’d just wait around for the next amnesty, which is the guaranteed outcome.”

Krikorian believes there is zero likelihood that two million people will be deported. That seems a reasonable assumption. And it’s true that implementing the E-Verify system, even if it overcomes business opposition, lack of administration support, and perhaps court challenges, will take years. So it seems likely that a large number of currently illegal immigrants will just stay in the United States — still “in the shadows.”

The Senate never debated this specific issue. But Sen. Jeff Sessions, perhaps the chief opponent of the bill in the Senate, did bring up the question of what would happen to legalized immigrants when, six years after being awarded RPI status, they are required to renew their status for another four years. What if they’ve done something — they’ve committed criminal acts, or have been unable to meet the requirements for a job and income, for example — which means they don’t qualify for renewal?

“At the end of six years,” Sessions said during a May 21 Senate Judiciary Committee meeting on the bill, “a person who has been given RPI status is supposed to have a job, the — a full-time student or have a certain level of income, I understand that if they don’t, that they’re to be deported.”

“Yes,” said Democratic Sen. Charles Schumer, the leader of the Gang of Eight. “If they don’t meet the standards, there are lots of standards in there and there are exceptions. Some people can’t work. They may have an illness. They may have young children, whatever. But if they don’t meet the standards, they can be deported, yes.”

Sessions immediately noticed Schumer’s word choice. They can be deported — what does that mean?

“Can be, should be, are we going to deport the — that will be hundreds of thousands of persons,” Sessions said. “And I just would like to know with an absolute firm commitment and if that’s what your intention is. It seems to me that it’s unlikely in a real world that that would happen anymore, then we are unwilling now to deport people who entered 18 months ago illegally. We don’t deport them. So somebody has been here another six years and unemployed, now you’re going to deport them from the country?”

“Well, we’re going to be under a new system, just remember,” Schumer responded. Schumer said E-Verify would make it difficult for an illegal immigrant to find a job if his RPI status is not renewed.

Sessions was skeptical. “And so if a person — what if we have a recession and an unemployment rate even higher than the high unemployment rate today, so are people all going to be deported if you don’t have jobs?”

“If you look at the base bill, there are various exceptions,” Schumer answered. “Not everyone has to be working at that time, but if they can work and are not working, yes, they can be deported.”

Again, Schumer said can be deported. For his part, Sessions was not advocating mass deportations. He was just pointing out that there will be millions of people who will not fit into the Gang of Eight’s reform structure. And it seems very likely that they will remain in the United States in much the same status they are today — a situation Gang members have portrayed as intolerable.

And the problem could be even more serious. Krikorian points out that after the 1986 Reagan immigration reform, one of the men who would in 1993 take part in the first attack on the World Trade Center applied for amnesty as a farmworker but was turned down. “With no provision for removing people who were rejected, he just stayed.”

Now, there are serious questions about those who will just stay if another immigration reform law is passed. The Gang of Eight never answered those questions. And they remain unanswered today.