The lights never seem to be on and the shutters stay shut.

As the weeks pass by at the White House, the corner of the residence long used by first ladies remains dark, because this first lady does not really live in Washington.

Melania Trump vanishes from view for weeks at a time, holing up in Trump Tower in Manhattan or in Florida, where she can lie low at Mar-a-Lago. Administration officials say she is at the White House more often than the public knows, but when exactly, and for how long, these officials will not (or perhaps cannot) say for certain.

It’s like having Greta Garbo as first lady.

Mrs. Trump is expected to reappear in the capital on Thursday to unveil a postage stamp honoring Barbara Bush, the former first lady, and to attend a ceremony for military mothers. But two people with knowledge of Mrs. Trump’s schedule said she had spent fewer than 14 days at the White House since her husband was inaugurated 108 days ago. Others say even that is a generous estimate. Officials in the East Wing and West Wing declined multiple requests for comment for this article.

That the first lady’s whereabouts is among the most sensitive of subjects in this White House only adds to the intrigue.

“We haven’t seen such a low-profile first lady since Bess Truman, and that’s going way back in living human memory, nearly 80 years ago,” said Katherine Jellison, a historian at Ohio University whose research has focused on first ladies. She said that, like Mrs. Trump, Mrs. Truman spent much of her time running back to “her home base whenever she had the chance.” (In Mrs. Truman’s case, that was Independence, Mo.)

“She just kind of liked her own private world,” Ms. Jellison said.

The same is true of this first lady. She has hired staff to work for her in the East Wing, but she rarely goes into the office. Even regulars at Mar-a-Lago say they don’t often see Mrs. Trump around the premises.