The discovery of 2,500 pounds of cocaine in the Valentine's Day cargo of a Colombian Boeing 747 in Miami last week was a major coup for the U.S. drug-enforcement effort. It was also an unlikely find for U.S. Customs Service agent Sidney A. Reyes.

Reyes is, after all, a suit-and-tie administrator now, the agency's district director for Washington. He was in Miami for a training course -- supposedly learning new drug-detection techniques, not showing his Florida colleagues how to do the job.

But there he was, dressed in jeans and climbing atop stacks of boxes, armed with a metal probing rod and the diligence of a hungry rookie. And for Reyes, last week's seizure "was like icing on the cake" of an up-through-the-ranks career filled with unlikely, headline-making airport drug busts.

"It was exciting, because I did it as an administrator," Reyes said in an interview this week. "I had a chance to take off the tie, put on some dungarees and get back into the trenches, so to speak."

When Reyes talks of his meteoric career, it is almost as if he is awed by it. Gazing at the numerous awards that line the wall behind his desk, Reyes said, "I'm exceptionally proud. Hopefully, it inspires a lot of other people. No district director has ever made a seizure that I can recall, so I'm making history again."

Reyes' story is all the more unusual since he barely made it into the inspector ranks to begin with. Born in Harlem, N.Y., he spent three years in the Marines and five years with the Postal Service before joining the Customs Service in 1959 as a grade-2 clerk in South Ferry, N.Y.

From the beginning, Reyes wanted to be an inspector, he said. But there were obstacles: There were few black inspectors and the tough inspector's examination was geared toward college graduates, not high school graduates like Reyes. He failed the exam "a couple of times" and then, by studying at night, passed it in 1967.

Reyes suspects that his struggles may have made him a better student. "I came up from the ghetto," he said. "It was a great achievement for me just to get in. I just absorbed everything." Even now, more than a decade since inspectors' school, he prefaces some remarks by saying, "Well, they taught us in school . . . ."

Reyes made his first drug seizure in his first week on the job. Stationed at New York's John F. Kennedy Airport, Reyes became suspicious of a young Jordanian who had helped another passenger who spoke no English understand how she was supposed to open her luggage for inspection. But when the man's turn came, he balked and protested.

Searching the man's luggage, Reyes found a false bottom that concealed 26 pounds of hashish, then worth about $50,000 on New York's streets.

Reyes made headlines in New York as the rookie who had nabbed a big one. "I got some harassment from my colleagues, who thought I was just a lucky stiff." But Reyes set the record straight for them a few days later. He made headlines again with another drug seizure. And he still had less than two weeks on the job. This time, though, it required some skillful interviewing.

"It was warm, but the passenger was wearing a raincoat," Reyes recalls. "He was sweating . . . . They told us in school to look out for anything unusual, like someone coming back from the islands with a fur coat -- it just doesn't jibe."

While searching the man's suitcase, Reyes noticed a pair of baby shoes and asked who owned them. The passenger nervously replied that they belonged to his son, who was still in Argentina. "Then why are you bringing them to New York?" Reyes asked, and the man stumbled to find an answer.

Reyes put the shoes aside and continued his search. No false bottom in the suitcase, no narcotics hidden in any of the usual places. Finally, he turned back to the shoes and asked brusquely, "What are these shoes for?" The passenger replied, "For my daughter."

First a son, then a daughter. It was enough of a discrepancy for Reyes to take the passenger aside and search him. The search turned up five pounds of heroin strapped around the man's waist underneath the raincoat.

It sounds like good detective work, but Reyes shrugs modestly. "They taught us in school to look for anything unusual." Drug dealers, he explained, will often throw an odd assortment of clothing into a suitcase that has no explanation: a bearded man carrying shaving supplies, a passenger with baby shoes, but no baby.

Since becoming an inspector 18 years ago, Reyes has seen many changes in the job. Jumbo jets now unload more than 300 passengers at a time. Airport customs agents can make instantaneous criminal-background checks on passengers, via computer terminals. And the old "assembly lines" in which every passenger had his bags inspected have been replaced, with one agent screening passengers who may proceed without a search from those directed to a line for inspection.

But customs inspection still comes down to that one intangible skill, perhaps best defined as intuition. "Some people call it a sixth sense," Reyes said. "We like to call it common sense."

"For example," he said, "if someone goes skiing in Switzerland, they come back with skis. An unemployed taxi driver going to Bogota for a weekend, that should tell you something is wrong. . . . . You see someone wearing a very expensive suit and a cheap watch -- you know someone just gave him the suit to look like a businessman."

From his perch at Dulles Airport, Reyes now oversees 57 employes and customs operations at all airports in the Washington metropolitan area, the most politically sensitive of U.S. Customs districts. Here, more than elsewhere, customs agents must perform a delicate balancing act -- securing the capital's airport borders while not disturbing a political and diplomatic caste system built on huge egos and the prerogatives of diplomatic reciprocity.

"The Washington area is unique because of the diplomatic climate here," Reyes said. "Diplomacy and courtesy are a must."

The job also has given Reyes the opportunity to travel abroad with two presidents. A customs inspector flies aboard the chartered press plane to facilitate customs and baggage checks when the presidential party returns to the states. Reyes has traveled with President Jimmy Carter to Poland, Egypt, Israel and Guadeloupe, and with President Reagan on his trip to China, among other places.

Reyes sees his job as contributing, in its small way, to solving the most pressing problems he saw while growing up. Every airport seizure he makes keeps drugs off the streets of New York.

Yet he makes no pretense that he or the hundreds of other customs inspectors can do anything more than make a dent in illegal drug smuggling. "As you know, smuggling is one of the oldest professions -- I won't say what the other one is. We think we're doing a good job. But there will always be people trying to bring in narcotics -- sometimes a 747 full of it."