When an airplane flew into the Capitol’s restricted airspace last Wednesday, some evacuated the Capitol — and some didn’t.

And that largely depended on who knew about the incident and who was kept in the dark.

Technically, no one was required to leave. The plane’s intrusion raised the Capitol’s alert level to Code Orange, the step before a mandatory, full-out evacuation.

In a matter of minutes, Capitol Police cleared the plane and canceled the alert.

But by that time, some of those who knew about the alert left pre-emptively, encouraged by some police officers to get ahead of an expected mass exodus.

That knowledge was parsed out with no discernable rationale: Senate Majority Leader Harry Reid (D-Nev.) announced the situation on the floor, while House leaders kept it to themselves.

Senate Sergeant-at-Arms Terrance Gainer said the decision to announce the alert lies with the Members. And overall, he contends, the incident was handled well — there was no chaos and minimal evacuation.

But an “after action report” on the incident will look into reports that some police officers told staffers and visitors to evacuate the Capitol. If that turns out to be true, he said, the department will have to address it with more training.

“To the extent possible, people should not self evacuate and officers should not opine that you should evacuate,” he said. “But I can hardly ever fault police for exerting their judgment.”

Majority Whip James Clyburn (D-S.C.) was one of the Members to leave the building, passing right by actor Tom Hanks and Speaker Nancy Pelosi (D-Calif.), both of whom spent the four minutes of the Code Orange at a reception for the HBO miniseries “John Adams.”

But while Pelosi stayed put, her staff was on alert, said spokesman Nadeam Elshami. As for why Pelosi didn’t make an announcement, Elshami said decisions concerning Members’ security are made by the Sergeant-at-Arms and “not the leadership.”

“Members of the Speaker’s security team were fully informed, and the situation was being managed by the Sergeant-at-Arms and the Capitol Hill Police,” he said.

But Clyburn and his office still left as a precaution, said spokeswoman Kristie Greco. Along with staffers from other offices, the group numbered about 25.

“Our office is essentially in the center of the building directly below the Dome, and it’s an old building, so evacuating quickly and safely can be a bit of a challenge,” she said, later adding: “It ended up being a helpful drill. It’s always good to go though your evacuation route and be prepared.”

On the Senate side, the periodical press gallery evacuated the Capitol, along with others who had heard about the alert — which was effectively everyone because of Reid’s public announcement.

“As soon as he learned about this, he felt the only fair thing to do was to let people know,” said Reid spokesman Jim Manley.

Of course, certain Members are told at the outset, though Gainer declined to say who in particular is notified, citing security concerns. But the department raises or lowers the alert depending on a host of factors, including the plane’s size, direction and whether it has a flight plan.

And in such situations as Wednesday, the Capitol Police is in constant contact with other agencies, such as the Department of Homeland Security and the FAA.

Based on that, police decided that the situation merited a Code Orange, which alerts certain people to prepare for a possible evacuation, Gainer said. Different codes bring in different parts of the Capitol Hill community; it can sometimes include doorkeepers and tour guides.

And when there’s all that movement, sometimes staffers just figure out what’s going on. At that point, they decide to evacuate the Capitol on their own. Gainer expressed hope, however, that they would trust the department to make the call.

“In an ideal world, we’d like people to have a lot of information and temper that with personal experience and their trust in the police officers,” he said.

He cited Wednesday as a job well done; while there was an uneven evacuation, the situation was handled effectively and quickly, he said.

It’s a stark improvement from June 9, 2004, when lawmakers and staffers frantically evacuated the Capitol before Ronald Reagan’s in-state funeral because of a similar situation. A plane carrying then-Kentucky Gov. Ernie Fletcher had entered restricted air space, and a breakdown in communications meant that the Washington defense officials were unaware that the FAA has cleared the plane.

Wednesday proves that such breakdowns are in the past, Gainer said. But the department hopes to keep improving, he said, and will work off the report on Wednesday’s incident.

“The difficulty that the police department has, along with everything else, is to balance these air incursions — that happen with less and less regularity but with some regularity — and not press the panic button,” he said.