Yeah, they're not going to launch a Saving Private Ryan search for every US citizen in the country.

Actually, you are supposed to register if you live outside the USA with the State department.

Actually, no. There is no "supposed to." You can register with the State Department if you live outside the US, just like you can register with the US if you travel outside the US. The web site is http://travel.state.gov/content/passports/en/go/step.html .

Do you really think that every one of the millions of American citizens who travel or live overseas each year is supposed to tell the US about their current location?

Do you really think that every one of the millions of American citizens who travel or live overseas each year is supposed to tell the US about their current location?

I didn't presume there was such a requirement, but I also wouldn't have been surprised if the fine print somewhere made it a selectively enforced requirement. It doesn't strike me as more or less onerous than some other actions that are currently required.

There is no "supposed to."

I think you are correct, but when an official government agency "encourages" a certain behavior, it's not too much of a stretch to presume that one is "supposed to" do it:

  We encourage all U.S. citizens travelling or residing 
  overseas to notify the closest U.S. Embassy or Consulate of 
  their contact information so that, in an emergency, we can 
  contact them to provide important safety and security 
  information, or support during a crisis.

https://step.state.gov/STEP/Pages/Common/FAQ.aspx

What would your guess be as to the chances that some sort of registration will be made requirement in the future?

"Supposed to" implies either a requirement or a moral obligation. There is neither. "Should" doesn't have that same sense. Perhaps the word you meant? It would very much surprise me if that fine print exists as I looked for terms like that when I moved overseas, to make sure everything was legitimate.

Such a requirement is very unlikely for travelers. It's just not practical given how often plans can change. (I visited a salt mine in Austria. To my surprise we crossed into Germany during the tour. Would I need to tell the US about those 20 minutes in Germany, and when?) How much would it cost to run such a system? What happens if the system breaks - and we know the foreign tourist tracking system breaks? What are the advantages to the US?

It's more possible for permanent residency. But the US doesn't even require that of domestic citizens, despite the clear advantages there would be to know where people live. So I don't think it's going to happen any time soon. And what would be the penalty of getting it wrong?

When a citizen has made their way to usually the largest, most well connected city in the continent already it's not much of a rescue.

And yes they were expecting a Saving Private Ryan moment.