For a while anyway, it looked like it might be the solution to a mystery - maybe even the missing 18 1/2 minutes from the Nixon Watergate tapes. Implausible. Unlikely, of course. But then so was Watergate.

A called, identifying himself as a guard in a building at 18th and G streets NW, phoned the city desk of The Washington Post Wednesday morning to report that workmen in the building had found some tapes marked "Watergate," and that security men in the building had them.

The day city editor, who knows that mighty oaks begin with tiny acorns dispatched a reporter, Alfred E. Lewis, to check it out. Lewis, who was the first reporter inside the Democratic National Committee headquarters in the Watergate after it was burglarized almost six years ago, knows something about acorns and oaks, too. He went to 18th and G to see for himself.

He found no guards in the building. The building manager told Lewis the guards didn't come on until 5 p.m. Lewis found some workmen on the 12th floor, but no one plastering as The Post's caller had said.

Lewis returned at 7 p.m. The guard he interviewed knew nothing. Lewis went to the 12th floor to see for himself. Ceiling tiles were out. Renovations were under way. Cleaning women sent him down to the eighth floor, where the Secret Service has its offices. The agents there knew nothing, they told him.

But Lewis still didn't give up. Yesterday, he called Jack Warner, director of the Secret Service's office of public affairs. Yes, Warner said, he had the tape.

Only it wasn't a tape. It was a film. Another reporter arranged to look at the film. It's a film, all right, sixteen millimeter and in color.

According to Warner a workman on the 12th floor was moving a tile when the film fell down. It was a white box that had written in ink: "Names (SSA) XXJS3086 60 sec. 5400 K." Then in pencil, the box said: "Nixon Watergate 1974." On the other saide, in pencil again, it said: "Private Use Only."

The workman brought the film to the Secret Service, where, Warner said, "people from the intelligence section" screened it. It was a television film clip which said, "Jose needs a job, just like John," and appears to have been made for the Equal Employment Opportunities Commission, which used to occupy the offices on the 12th floor.

"We didn't get too exercised about it," Warner said.

So now Warner has the film, which has nothing to do with Watergate - so far as anyone can tell. What will he do with it? he was asked.

"Probably throw it away," he said, pausing for a long moment. "But if I do that, then someone will say we destroyed a Watergate tape. Really, the best reason to hold on to it is because of people like yourself," he told his questioner. "I figure I'll hold on to it for awhile. With the cynical minds around today, nobody would believe it."