An unusual object reported at a Schaumburg Metra station.

A backpack along the side of a road in Wheaton.

Another backpack near a New Jersey train station. That one, found Sunday night by two homeless men, turned out to contain bombs. They alerted police and “probably saved hundreds of lives,” Elizabeth, N.J., Mayor Christian Bollwage said.

The men did what police want us all to do.

It’s a fact of life in the post-9/11 world that abandoned packages are worthy of suspicion. Whenever the “See something, say something” signs and announcements begin to feel like no more than the wallpaper of urban life, something like the weekend bombings in New York and Seaside Park, N.J., and the attempted bombing in Elizabeth shakes us out of complacency.

Perhaps that heightened awareness was responsible for Monday’s report of a suspicious object at the station in Schaumburg. A Metra police canine unit and Schaumburg officers responded and, while trains were held back from the station, determined the gas can left near the ticket area did not pose any danger.

The people who reported it did the right thing, authorities say. No one should be deterred by concerns about being embarrassed if a report leads to nothing.

“We encourage people who see suspicious packages to call 911,” Schaumburg Police Sgt. Christy Lindhurst said, adding it’s best to err on the side of caution.

What’s suspicious? It’s often in the eye of the beholder, says the federal government , which relies on people to recognize what’s out of the ordinary as they go about their routines. Examples on DHS.gov include vehicles in odd places, suspicious bags, a door open that’s usually closed, or someone very curious about building security or operations.

In Wheaton, sadly, the danger was not to the public. A newborn girl’s body was found in August near a backpack that had been noticed along the road the day before. Police have not said whether earlier discovery could have made a difference for the baby’s survival or in locating the person who left her.

In the weekend’s bombings, observant people are credited with three instances of disrupting a terror attack.

Along with the two men who reported the Elizabeth, N.J., bomb, a resident noticed one of the pressure cooker bombs in Manhattan and a bar owner found the suspected bomber sleeping in an entry.

What if you’re the one who forgetfully left your backpack in a crowded mall? You might head off a police response (and maybe get it back!) if you report its location to security, Lindhurst said. Sometimes, a suspicious bag is just a lost backpack. But we have to be prepared for the times when it’s not.