They cruise along silent streets, glimpsing suburban life through open curtains. A cocktail party at one house, the kids glued to the television at another. A woman sitting behind a baby grand piano in the picture window across the way.

And then there are the empty houses, the ones cloaked in inky blackness. The car slows to a creep. They peer into the shadows. They strain to see around the shrubbery.

No one in sight. They move on.

Phyllis and Leigh Conrad patrol the streets of the Brookhaven/Forest Villa area of McLean one night a month. They are part of a neighborhood watch group organized about two years ago in reaction to an escalating crime rate in the community.

Neighbors take turns patroling the streets that wind among the 200 houses in the community. The cars are equipped with citizens-band radios monitored from yet another neighbor's house that serves as a command post.

Last Saturday night, all the Conrads caught on their patrol was a lot of static on the CB from other neighborhood patrols.

"We've never really seen anything (criminal)," said Phyllis Conrad. "A burglar would have to be pitifully bad for us to catch him," she admitted, but she added, "The idea is for us to be seen rather than for us to see anything."

"I don't know how much good the patrols really do," she said, but she believes they may deter crime indirectly. Word spread through the community schools that the neighborhood was under citizen surveillance, and since then there has been less juvenile mischief in the area, she said.

"Of course, we notice the Neighborhood Watch signs at the entrance to the area have been defaced," said Leigh Conrad, pointing out a once-blue sign now spattered with gray spray paint. "Someone doesn't like us."

Earlier in the evening, Conrad Marshall and Elizabeth Thomas rumbled over gravel country roads on another neighborhood-watch patrol, this one in the Potomac Ridge Road area near Great Falls. Their patrol, barely a year old, is less formal than McLean's.

The owners of the 23 homes in the spacious, wooded subdivision drive over the backroads sporadically. Sometimes they use yellow, squad-car-type dome lights and shine flashlights into the thick undergrowth.

"We're not vigilantes or anything like that," said Marshall.

In the summer months the resident patrols often surprise teen-agers parked on the isolated roads, he said. On wintry nights, they scare up little more than a raccoon or two.

But they take their patrol seriously, saying the effort has stopped much of the vandalism and other crime that plagued the affluent, secluded neighborhood last winter.

Three houses had been burglarized within weeks of each other. Someone set fire to a row of 14 neighborhood mailboxes. Peeping Toms had been seen by several neighborhood women. Young hoodlums were chopping down trees and fences, creating drag strips for four-wheel-drive jeeps and trucks.

"We were fed up with all these intrusions," said Marshall.

But the Potomac Ridge Road group also has discovered a side benefit to its program.

"It's helped us get to know our neighbors," said Thomas. "It made people more friendly and neighborly. It's some return to the old days."