For Mark Siegel of Hamilton Jordan's staff, Thursday lunch in the White House may be a "picnic" - Mexican food from the Mess shared by several associates who sprawl on the floor of his office.

Washington's birthday offered Tim Kraft, President Carter's appointments secretary, a rationale for escaping for lunch. "So I went to a Chinese restaurant a Secret Service man recommended," he said, "but it was closed for the holiday. That was my big deal lunch."

Another staff member, Greg Schneiders, doesn't even eat lunch. "I gained 20 pounds during the campaign," he said, "so now I'm playing racketball instead of eating."

So far, those who work in the Carter White House have shown little interest in The Washington Lunch. "Today I had a sandwich and a salad at my desk in the midst of a meeting," said a senior State Department official. "I don't know where people want to go," said a speechwriter who works in the Executive Office Building. "But I see a lot of people carrying trays to their offices."

But out there, in the land of vichyssoise and credit-card feasts, the maitre d's are waiting. At Sans Souci, the luxury dining place nearest the White House, Paul DeLisle said a few officials from the new administration had come in.

"There have not been many yet," he said. "It will be some time before they get organized and start coming out. That's normal procedure. It's been like that for 24 years since I've been in Washington. In another four or five months I'll start to see it."

DeLisle left the telephone momentarily to bid au revoir to Mary Finch Hoyt (Mrs. Carter's press secretary), and then returned. "Mrs. Hoyt is a customer from before," he continued. "So is Mrs. (Bess) Abell (press secretary to Vice President Walter Mondale's wife, Joan) and Mr. (Jack) Watson. There are two young men here today. I know they are from the Carter administration but I can't identify them yet. I have to begin to put names on the faces."

Duke Zelbert's is a few blocks further from the White House, but it has long been popular with politicans. At the moment, though, there is a void.

"I had some people in for dinner last week," said maitre d' Mel Krupin. "They had been asked to the White House, but not 'til after dinner. Other than that nobody is coming around, nobody's spending. Tips are down. But you can't blame the Carter people. I don't know who they are. Nobody knows who they are."

Carter people have been out, of course. Tim Kraft has been seen dining at night at the Court of the Mandarins. Schneiders and his wife will stop some evenings at the Old Ebbitt Grill. Some staff members have gone to Blues Alley for steaks and music. Others mention the Two Continents in the Washington Hotel, the Hay-Adams.

But these are isolated instances and there are indications that the expense account restaurants may have a longer wait than they anticipate this time around.

"Out people don't identify with the basic Sans Sounci lunch," said one official."They're more comfortable at the Steer (the Black Steer, an informal steakhouse just north of the Sans Souci).

"It's not just the time factor that keeps people from going out," added Schneiders. "A lot don't drink during the workday. That takes some of the glamor out of a French lunch."

Most of those who do go out "try to stay as close as possible to the White House," Seigel said. For some that means sandwiches at Kay's Sandwich Shoppe on G Street NW, or carry-out orders from Loeb's on 15th Street NW.

Tim Kraft has been to Loeb's, but Walter Loeb, the owner, didn't know it. "We've delivered orders to the Executive Office Building" he said, "to Vice President Mondale's office. We put peanut soup on the menu for the Inauguration and it's moving, so I've kep it on. New faces? I can't really say I've noticed many. But we're ready for them."

There have been reports that a bar on H Street called The Class Reunion is the "in" spot for those few Carterites who don't head home to their families immediately after work. A White House source confirmed that is one place. He indicated that Sarsfield's, on L Street NW near Georgetown, was another that had escaped media attention. "It's a hangover," he said, pleased with the play on words. "It was sort of adopted by the field staff during the campaign. I don't know if they eat there, but they sure close it a few nights a week."

Meanwhile, back at the White House Mess, there has been at least one significant change. During the Nixon and Ford administrations the official soft drink sold by vending machine was Pepsi-Cola. During that time Pepsi Board Chairman Don Kendall was active in Republican Party activities. Now there is not a Pepsi for sale. Coca-Cola, the pride of Atlanta, Ga., and its related product, Fresca, fill all the slots.

Lucky Coca-Cola. In these early days of the Carter administration, soda pop is selling faster than the vintage wine available around the corner.