Sometimes the most incredible things turn up in the trash. Robert Ellis Smith, publisher of the monthly newsletter Privacy Journal, keeps what he calls a list of ''Great Moments in the Storage of Sensitive Information.''
Here is a sample:
* A Waco, Texas, couple was indicted in October 1978 on drug charges after police found syringes, needles, and traces of drugs in plastic bags waiting for garbage pickup.
* In June 1979, two boys playing outside their West Orange, N.J., elementary school found their report cards and other school records, including student progress reports and psychological test results, in a cardboard box sitting at the edge of the street with rest of the school's garbage.
* In 1980, Milwaukee police sorted through a man's garbage for a month until they had collected enough evidence to charge him for involvement in an illegal gambling operation.
American law enforcement personnel ranging from the Federal Bureau of Investigation to small-town municipal detectives have frequently used information obtained in trash cans to assist in criminal and other investigations.
But garbage snooping is not just a law-enforcement activity.
In 1975, a reporter for a weekly national tabloid sifted through the curbside trash of then-Secretary of State Henry Kissinger, which helped raise such rummaging to a distinct science - ''Garbageology.''
According to corporate security experts, with the current premium on information in competitive industries, the practice has become one of an array of techniques used to gain a leg up on business competitors.
In the corporate world, garbage snooping is widely regarded as unethical. But that hasn't prevented it.
One case of corporate ''garbageology'' occurred in 1978 in Oakland, Calif., where a distributor for the Advance Machine Company rummaged through a dumpster used by his competitor, the Tennant Company, a floor maintenance firm.
The AMC distributor discovered, among other items, the carbon paper used in the typing of lists of Tennant's prospective customers.
The trash monitoring tactics were discovered when a former AMC employee related the story during a job interview at Tennant.
Tennant sued Advance Machine Company last year and was awarded $500,000 by a jury. The judge later reduced the award to $100,000. Tennant is appealing the judge's action.
At the core of the case is the question of whether Tennant has a reasonable expectation of privacy for the garbage in an office-complex dumpster.
One of the keys, according to privacy experts, is where the trash was at the time of the rummaging. If the garbage is at curbside, in effect on public property, and waiting to be picked up by sanitation workers, the experts say it is fair game.
If the garbage is in a building, federal agents would need a search warrant and a business competitor would need the owner's permission before rummaging through the trash.
The Fourth Amendment to the US Constitution guarantees ''the right of the people to be secure in their persons, houses, papers, and effects against unreasonable searches and seizures. . . .'' That is, it guarantees the right to be secure from government searches of one's home or one's car, experts say.
''If you throw something in the trash and there is no illegal entry involved in getting at the trash, then a person doesn't have a reasonable expectation of privacy,'' says Norma Rollins, director of the American Civil Liberties Union's privacy project.
''I think it is unethical to do it,'' says Mr. Smith of the Privacy Journal. ''But I don't think you have a valid claim to privacy if you have already put it out in the garbage.''
He quotes a Federal Appeals judge in Chicago as saying in April 1978: ''It seems more prudent to put only genuine trash, not secrets, in garbage cans.'' The judge made the comment in upholding an FBI search of a suspect's trash, which linked the suspect to the $3,000 theft of coins from nearby banks.
Although law-enforcement agencies will probably always use garbage as a source of information when necessary to track down criminals, not all competitive businessmen feel a need to resort to snooping through garbage.
''We don't go through people's trash,'' says Leonard Fuld, president of Information Data Search Inc., a Cambridge, Mass.-based firm specializing in competitor intelligence. ''We've never done it. First of all, it's too dirty. But also it is just not that productive. I believe very sincerely you don't have to stoop to anything illegal or unethical to make sound business decisions.''
But, he adds, ''People will do anything sometimes to get information.''