By Jim Williams

CHICAGO (CBS) — Shortly after the Boston Marathon attack, President Obama urged Americans to remain vigilant.

You know the saying, if you see something, say something. But what does that mean?

Jim Williams went to find out in this original report.

Security expert and former Chicago police commander Neil Sullivan was at the wheel in a North Side neighborhood showing CBS 2 what to see and when something looks suspicious.

Sullivan said that, “If a citizen were looking out their window or walking down the street and happen to see someone come and put a package in the newspaper box as opposed to taking a newspaper out of the box,” then it’s best to report it immediately.

Going to a neighborhood festival this summer where there are lots of garbage bins?

“You have to take a close look as to the types of things people are putting in there,” said Sullivan.

Simple food wrappers are harmless. Be concerned about very large containers.

Ever seen workers around hazard signs?

Make sure their vehicles are clearly marked “to see if indeed they look like they represent some sort of government agency or water department or telephone company,” said Sullivan. And if it is unmarked, “I take a close look at it and make note of the licensing plate.”

Neil Sullivan recommends putting your smart phone to use to prevent terrorism to “put down a description of their vehicle, put down their license plate number and put down the direction they have left the scene.”

In Boston that law enforcement officials are asking the public for any videos or photos of the area around marathon’s finish line.

Now, Neil Sullivan has worked a lot of big events: the World Series, President Obama’s victory celebration in Grant Park, NATO and he says location is very important. A gym bag left in a health club locker room is one thing. Could be something else in a porta-potty or garbage bin.

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