David Rostan, a second-year law student in New York City, came home one night in March, opened his mailbox and found a calling card. Written on it was the message: "Please call me about Nicaragua. This will be a friendly chat." Underneath was the name of a special agent of the FBI. Since Rostan has never been to Nicaragua and is not particularly impassioned about events in Central America, he couldn't figure out at first why he had become a person of interest to the FBI. Then he made what might be the connection. A few nights before, a student at the law school who had recently returned from Nicaragua was being given an informal dinner at the school. Rostan was there late, working on a project. He was hungry, so he walked into the room where food was being served and the traveler was being asked about her trip. Rostan added a few critical questions of his own. The night he found the FBI calling card, Rostan did not sleep at all well. But he has not called the special agent. "I don't like the furtive way they did that, slipping the card into my mailbox," he says. "And how did they get my address? I'm not in the phone book." Rostan would also like to know what business it is of the FBI if he chooses to listen to somebody talk about her trip to Nicaragua. The FBI's fingering of Dan Rostan may indicate a new stage in the bureau's diligent monitoring of Americans who might be suspected of disagreeing with the Central American policies of the only president we've got. Until now, the FBI has focused primarily on contacting and trying to question those curious Americans who have actually been to Nicaragua or El Salvador to pick up firsthand impressions. In some cases, special agents have also visited the landlords or employers of these doubting souls -- just a friendly advisory service provided by the bureau to patriotic American bystanders. The Center for Constitutional Rights in New York has been compiling a lengthening list of Americans who have found the FBI waiting for them when they come back to the home of the free. One of them, a Detroit woman who had been in Nicaragua, called Michael Ratner, an attorney for the Center, when she got an FBI calling card. Ratner telephoned the special agent, who said, "We try to interview everyone who takes a trip to Nicaragua. We do it for positive intelligence gathering." There are some crabbed observers, however, who figure that this operation involves quite another dividend for the FBI: putting a big chill on the plans of other critics of the administration to travel to certain countries. No one really wants to be targeted by the cold eye of the FBI. Jerry Berman, legislative counsel of the American Civil Liberties Union and an expert on FBI surveillance, is known among both friends and critics of the bureau as a man who never cries, or even whispers, "wolf." His manner is low-key and his language careful. I asked Berman what he thought of this rising FBI interest in certain returned travelers. "I'm not speaking for all the cases," Jerry Berman said. "But it does appear -- based on the scope and range of people being interviewed by the FBI -- that the bureau is, in some cases, going beyond the scope of its legitimate counterintelligence mission. The FBI is authorized to investigate persons reasonably suspected of engaging in clandestine intelligence activities on behalf of a foreign power, and of violating United States laws. "But the FBI is not authorized," Berman continued, "to investigate people who simply agree with the policies of a foreign power but are not under that government's control and do not violate our laws." Jack Novik, a staff counsel of the ACLU, has also studied the FBI carefully. He believes the Reagan administration "may be gearing up to protect itself against rising criticism of its Latin American policies. In our history, from the Alien and Sedition Acts of 1798 on, political surveillance has frequently focused on critics of foreign policy. I'm just expressing a concern, but the pattern here could indicate that a political intelligence apparatus is collecting the names of critics of the administration's Central American policies for later use." But Attorney General Edwin Meese has assured us that the FBI does not engage in political surveillance of anyone unless there is some link to criminal activity by the person being watched. The attorney general, however, appears unaware of the FBI's most obsessive tradition. Just as Joe Hill said he had never died, neither has J. Edgar Hoover.