Eighteen months ago, furniture movers left a seven-foot sofa on the elevator of a D.C. government building on H Street NW. It hasn't been seen since.

At the International Office Building on F Street NW, an office worker watched a man she thought was a repairman remove three air conditioners from windows and walk away with them.

Two years ago, someone walked into the reception area of the E. F. Hutton Co. office in International Square, mumbled something about "cleaning a rug," rolled up the $5,000 Persian rug and walked away with it.

Office thefts are big business in downtown Washington. In 1979, some $4.4 million in cash and property was stolen in 11,765 thefts in District offices, according to the police department. That represented a 13 percent increase in the number of thefts over the previous year -- and a 57 percent increase in the value of the items stolen.

Thieves pose as newspaper boys, messengers and repairman to convince people to let them wander through offices. Once inside, they take anything that's not nailed down -- typewriters, purses, wallets, coats, furniture, small office machines or postage stamps.

A skilled thief can earn $400 a day stealing wallets and cash, estimates Sgt. William B. Novinski, commander of the theft unit for the police department's 2nd District, where many of the offices are located.

Thieves use a variety of ingenious techniques.

Caught in a building near the White House, one District youth looked out a window, pointed to a tour bus and convinced the office employe that he was a student with the group and had been sent inside to find a good angle for a photograph of the White House. The youth was later arrested and found to be responsible for about 100 office thefts.

A thief at the D.C. Probation Office on 4th and F streets NW was startled by a female worker as he walked out of an office with a purse. He told her that he was returning the purse to his sister who worked in the office. As she went to look for the fictitious sister; the man stole her purse, too.

Another thief posed as a Coast Guard officer and stayed inside government buildings when they were locked at night. A GS-13 manager at the Law Enforcement Assistance Administration was found to be stealing office typewriters to help make his alimony payments.

But along with the more bizarre thefts are thousands of wallets and purses, stolen out of desk drawers and jackets slung on the backs of chairs.

Yet, while office thefts have been on the rise, police have been cutting back on their enforcement effort.

Fewer than a dozen detectives from the 1st and 2nd districts handle all the office thefts in the city. A year ago, Novinski's unit had 14 detectives; half of the squad, he said, has since been reassigned to work on violent crimes. Over the past year, the 1st District's squad has also been cut back from six detectives to two.

The squads work primarily during the day, Novinski said. They check pawnshops for stolen goods, stake out office buildings, plant undercover officers in buildings and use electronic surveillance equipment, among other methods, to try to catch the thieves.

"Very few major fences [persons who receive stolen property] have been arrested because the thieves are reluctant to tell who their fences are, said Novinski. "But there are too many typewriters stolen in this city to be fenced to a college student or a friend."

To guard against the thefts, police recommend that workers place valuables in locked desk drawers and ask repairmen for identification and strangers where they are going.

"People think they know what thieves look like," said Detective Joseph Boomer, head of the First District's theft unit. "[But] the office thief looks like everybody out there."