It seems to me that if I were doing something like this, I'd have some way of indoctrinating people without them knowing it. Some information or phrases that seem very commonplace and non-important, but will trigger certain responses when they're seen or heard again.

We've already got quite a few that are easily spotted by an enemy. When the national anthem plays, what do you do? It's pretty easy to spot people who don't know.

In fact, we already use this to spot enemy agents. They will react in odd ways to things that we think of as commonplace... Words and phrases that mean something in the culture, but on their own are meaningless... Or just have changed usage.

The only thing that makes me think the above is NOT what they used is that they said they would have to find a way to vary it. It takes time to make the above happen, but varying it isn't a problem other than that.