Let's say you are a terrorist and you want to know which line or lines of your communications are safe and which ones have been compromised. Let us assume further that you have three lines.
What you do is cook up three different communications. One of them, say for line 1, would hint at an unspecified "Europe" attack; line 2 would suggest an attack on Mexico City; line 3, an attack on Sidney, Australia. You then sit back and watch government reactions.
The State Department warning for Europe issued today would prove beyond a doubt that line 1 is not reliable. Since there is no unusual action in Mexico City or Australia, lines 2 and 3 would be considered safe.
The terrorist leadership might decide not to close down communication line 1; rather, they could continue to use it to mislead and confuse their enemies.
Communications tests can also serve to discover traitors:
For years, I worked for the leadership of a state legislative body. That leadership (let's say Democrat) knew the Republicans had a spy in the Democrat party caucus, but could not identify him.
I implemented the following tactic: in a Democrat caucus meeting, the leadership handed out to each member a "secret" report containing public opinion poll results on pending legislation. Each report had a different unobtrusive mark at the bottom.
As anticipated, the spy gave his copy to the Republican leadership, which in turn copied and distributed the report to each Republican.
Unknown to the Republican leadership, we had a spy in their caucus. Our spy gave me a copy of the leaked report. I looked at the mark, went down my checklist of which Democrat had received which marked report; in a minute, we had our man.
Similarly, the terrorist leaders may be sending out information that varies slightly to each of their operatives, then watching governmental reactions in order to determine which operatives are spies.
The way to counter such moves is to do exactly what the American government did not do. If it sincerely believes that a European attack is imminent, it should nonetheless widen the warning to other continents. That way, a terrorist test of its communications network -- and of its members' loyalty -- would fail to provide conclusive, useful results.