But what types of behavior are worth noting?
Retired Col. John Rogers, now a senior associate for the Web site TotalSecurity.US, told The Early Show co-anchor Harry Smith Thursday there are definitely some that matter more than others.
Among them: nervous behavior, seasonally inappropriate clothing, the delicate handling of packages, leaving packages unattended, and moving toward the center of big crowds.
"(People) need to be aware of their surroundings, in case they see any suspicious or unusual behavior," Rogers says. "Things like, if they notice someone not making eye contact, or if they happen to see someone who shies away or backs away any time a law enforcement official may be around. Things like, if they're wearing clothing that's inappropriate for the season, like heavy coats, for example, in the summer. They may be able to hide something underneath. Those kind of things. And just be aware of their surroundings."
In the London bombings, it looked as if some of the terrorists may have been fiddling with their packages, or been extra-protective of their packages, Smith notes.
"That, also," Rogers responded. "Obviously, with so many commuters you have backpacks and book bags and briefcases. Almost everyone carries something. But, if they happen to see something where people leave it unattended, maybe wires sticking out. Something that's that obvious, obviously we need to consider that and report it.
"If you think someone is suspicious, or they're acting out of the ordinary, you need to contact someone who works in the transit system, whether it's law enforcement, a transit policeman, or a conductor or someone like that, just to relay what you have seen; not to cause any fear, but just to report it."
Asked about the issue of where to draw the line on racial profiling, Rogers called it "the $64,000 question. Obviously, you know, they would draw some attention by certain individuals to look at them, but you must remember that it's not just always the Middle Eastern male. In the London bombings, for example, we've had East Africans. We've had Muslim converts who could be U.S. citizens who don't look like a Middle Eastern male. So, you've got to use some common sense and profile not what they look like, but what the suspicious behavior may be."