After he was arrested in Los Vegas, Nev., last Friday on charges of stealing more than $850,000 from the government, Transportation Department employee William C. Sibert was overheard on a telephone call telling someone, "They'll never find it."

The federal government went to court yesterday in an attempt to insure that it finds all of the money allegedly spent by Sibert in a two-month spree.

Justice Department attorneys asked that the court put Liens on Sibert's property and take other steps to prevent anyone from spending any of the allegedly stolen money.

In the suit filed in U.S. District Court here, the government outlined the progress it was making in determining how much money Sibert was able to spend before his alledged scheme surfaced.

So far, the government said, it has determined that Sibert had six checks totaling $856,557.72 sent to him instead of to the Atlanta subway system. The first check reportedly was issued on May 16 and was for $55,000, and the last one was issued on July 29 for $315,000, according to court records.

The government attorneys said that Sibert appeared to have spent at least $600,000 of the money "on such diverse items as a topless bar and IT&T stock" before he was caught. They said their problem was to trace the purchases to account for the money.

When he was arrested in Las Vegas, Sibert had $59,000 in cash with him. In addition, his checking account at Suburban Trust in Hyattsville contained slightly more than $100,000 according to FBI affidavits filed in court yesterday.

"That leaves $650,000 to account for," government attorneys said in asking the court to impound Sibert's property.

In accounting for that money, the government gprovide a list of what it had found so far.

According to the list, Sibert spent $60,000 for a new house in Clinton, Md., and then another $30,000 in improvements to it such as a swimming pool. He spent $9,000 more for furnishings, the list continued.

The next $18,000 was spent for appliances, stereophonic and other electronic equipment for himself and "friends," according to the government.

A 30-foot houseboat was purchased by Sibert for $30,000, and another $30,000 was invested in stocks and securities and similar items, the government said.

Attached to the suit was a list of 12 cars believed to have been purchased by Sibert for a total of at least $190,000. Federal agents said one of the cars alone, a custom-designed Lincoln Continental convtrtible, cost Sibert $35,000.

Government investigators said they found another $21,000 more in Sibert's credit union account at the Department of Transportation, where he was GS-5 financial assistant with the Urban Mass Transit Administration, and $65,000 in bank accounts of a woman friend of Sibert's.

The woman, Lois Beason, was arrested with Sibert in Las Vegas last Friday.

Also included in the government's list was an unspecified amount of money Sibert reportedly had set of aside for the purchase of land in Arizona and Florida.

The telephone call in which Sibert said, "They'll never find it," was overheard by a Secret Service agent, according to an affidavit filed yesterday with the suit.

Sibert is being held in Las Vegas pending his voluntary removal to the Washington area in the near future.