Chicago's wall of security penetrated by ... Commander in Chief

This city is on high alert after three terror arrests on the eve of the NATO summit, so average citizens are not supposed to get anywhere close to President Obama and the other visiting world leaders.

That's why there are a string of humongous dump trucks lined up outside the president's hotel to deter potential suicide bombers, dozens of Chicago police officers stationed on the street to ward off people who are not supposed to be at the downtown Sheraton, and metal detectors in the lobby to check hotel guests who do belong but could be a threat.

Sometimes, however, even the president simply wants to take an evening stroll -- and sends the Secret Service scrambling for a Plan B.

It was close to midnight on Saturday night when eyewitnesses say Obama simply left his safe and secure room upstairs and hopped on an elevator with his security detail to get a peek at the lobby.

Hotel guests and various tourists who were wandering the lobby in the highly unlikely event of bumping into the president let out shrieks and screams as the man himself suddenly walked off the elevator. He started shaking hands all around and even stopped to pose for  cell phone photos, including with one stunned guy who said he was celebrating a big day.

"Happy Birthday!" called out the president, who was casual in rolled up shirtsleeves.

Veteran Secret Service agents said privately that they had never seen the president take an unplanned walk around a hotel lobby. Even with the metal detectors in place, guaranteeing the people in the lobby had been screened, unscripted moments like this simply do not happen post-9/11.

The Secret Service agents did not seem bugged by the move, which various tourists said was refreshing for them.

Several Secret Service agents though  had to circle the president for safety and start demanding -- politely but firmly -- that individuals step back to give Obama space as word spread and the crowd started to build.

Earlier in the evening a media "lid" had been called for the night, meaning the president was not planning to make any more public appearances, so the protective pool of reporters that normally follows him was not in the hotel lobby chronicling the scene.

When a Fox News reporter happened upon the raucous scene of screaming tourists and walked up to a metal detector upon returning to the hotel, the president had now made his way to the metal detectors to shake hands with some of the Secret Service officers that handle security screening.

As the Fox reporter stuck out his hand to shake the president's hand, Obama shook and suggested with a smile there was no need to have the journalist empty the contents of his pockets.

"I know him he's ok, he follows me around -- let him through," the president said.

For the record, the president said it in a joking tone. And the Secret Service officers still made the reporter go through the metal detector.

After getting lots of grief following the president's recent visit to Colombia, the agency does not need any more headlines during a big trip.