For three months, more than 100 FBI agents staked out five sites in the Virginia countryside 60 miles south of Washington in a futile attempt to catch a Soviet spy.

But the espionage agent, suspected of buying military secrets from someone at a remote Navy base along the Potomac River east of Fredericksburg, never showed up at any of the "dead drop" zones that were watched around the clock by teams of government agents.

Details of the FBI operation became public yesterday after workers at the isolated Naval Surface Weapons Center at Dahlgren, Va., told reporters for a Fredericksburg newspaper that FBI agents had warned them to be on the lookout for someone who might be helping the spy. The workers were said to be alerted to watch for people with ties to Iron Curtain countries, workers asking for classified data they didn't need or colleagues who appeared to have suddenly come into a lot of money.

Personnel at the Dahlgren Center work on the development of submarine-based nuclear missles, satellite surveillance systems, electronic warfare gear and other Navy weapons spokesmen there said yesterday.

"It's normal procedure to try to alert [employes] to be aware of who might be the source of information leaks," said FBI spokesman Ed Gooderham yesterday.

The massive FBI effort began last March after Rudolph Albert Herrman, a former colonel in the Soviet KGB secret police, told U.S. authorities that one of his final assignments involved locating drop sites near Dahlgren.

Herrman, who posed as a New York-based free-lance photographer for 11 years, has provided "voluminous" details on KGB operating procedures the FBI said.

The Dahlgren episode proved less fruitful, the FBI spokesman said yesterday.

But while the agents have been unable to locate either a Soviet agent or a contact inside the base, the FBI said the exercise could be helpful in plugging any leaks at the installation.