In the community relations office of the Montgomery County police department's windowless, subterranean headquarters in the Rockville Mall are two posters.

One depicts San Francisco's Golden Gate Bridge against the background of a breathtaking sunset. The other offers the view of a towering, snow-capped mountain as seen from a deer-populated, wooded glen.

Lynn Marceau, the office's secretary, recalled that when her son visited her shortly after the police department moved into - and under - the mall three years ago, "he was here only a few minutes when he looked around and said, 'Mom, you need a window."

Her son's solution was the posters, crisscrossed with brown tape to simulate a window frame.

Come December, however, Marceau and the other 144 police officers and employees will be able to look out of real windows during work hours. Last week the County Council voted to approve purchase of a new building as the department's temporary headquarters.

The $2.1 million, two-story building now being completed at 2350 Research Blvd. in northern Rockville will house the department's headquarters until the Council-approved new County Office Building is built in downtown Rockville and ready for occupancy in about 1983.

For more than an hour at last week's session, Council members and Robert Lanham, the county's community and economic development director, discussed whether it would be cheaper to keep the police at the financially beleagured mall than to buy a new building.

Council members learned that it would cost about $1.4 million to either rent space at the mall over the next six years or pay debt service and other expenses on the new building for that time.

That information, coupled with the new building having enough extra space (about 8,000 square feet) to house other county offices, cleared the way for the 6-to-0 Council vote for the new building. Council member Neal Potter abstained.

Almost since the department moved into the mall, police officials and employees have complained of maintenance problems, poor ventilation, periodic infestations of insects and the morale-damaging lack of windows in their "bunker."

I'm very pleased with the Council vote," Police Chief Robert J. diGrazia said. "Citizen will be able to find us more easily, and we'll be able to see sunshine, snow, whatever the weather is. It'll make us all more productive."

The Council vote was bad news, however, for the mall's leasing agent and the city of Rockville.

Anthony Rolfe of Sulzberger-Rolfe, Inc., the New York City based leasing company, termed the move "an unwise decision made from the heart, not the head. It's not warranted by the facts."

After a February report by police and county officials recommended moving from the mall, Rolfe had offered a new lease and $75,000 worth of improvements to police headquarters at the mall's expense.

Last week Rolfe asserted that the police relocation, although serious, wasn't a "death knell" for the $14 million complex. Since its opening three years ago, the mall's vacancy rate has hovered around 30 per cent. Rolfe, however, said he has signed four new tenants recently and that only about 10 per cent of the Mall's store space is vacant.

Dan Hobbs, Rockville's assistant city manager, was also critical of the Council decision, stating that he wasn't "convinced every effort was made to keep them in the downtown "Rockville) area." Hobbs said the city will lose $12,400 in taxes next year because the county purchase of the Research Boulevard building will remove it from the tax rolls.