Why Salvador's missing persons vanish

''Missing: Orlando Maximiliano Pena Quintanilla, 16 years old, disappeared from his house Jan. 29 and has not returned. His father would appreciate any information as to his whereabouts.''

''Searching for: Adan Melgar Escobar, 40 years old, who disappeared March 27 on his way to work.''

''Found: Carlos Armando Aquino, 24, having disappeared recently, was found dead in a coffee finca (plantation) in El Salitre de Nejapa.''

The names and faces staring out of postage-stamp-size family snapshots sprinkled through Salvadoran newspapers change daily. They are known as the desaparacidos - people who have simply disappeared.

If they are found, it is usually too late. Then they are listed by the Salvadoran Human Rights Commission, the Central American University, and the United States Embassy here as ''violent'' or ''politically motivated'' deaths.

According to the US Embassy, there were 6,116 such violent deaths in 1981. The Central American University documented more than 13,000 in the same period.

The US says the discrepancy in the figures results from university and Human Rights Commission sympathy for antigovernment forces in El Salvador's civil war. The embassy bases its much lower figures on reports that it sees in the Salvadoran press.

The countercharge against the US is that its lower statistics are an attempt to play down the number of deaths to prove to the US Congress that human-rights violations in El Salvador are declining, and that the country meets US standards for receiving aid.

But almost all the statistics-keepers agree on the number of desaparacidos. One political analyst estimates between 30 and 60 people disappear each month. The Salvadoran Human Rights Commission lists 54 for January, 37 for February, and 69 during March.

How do people vanish into thin air?

A Salvadoran university professor who asked that his name not be used says that ''in times of 'total war' the term 'subversive' covers such a broad range, that anyone can fall into the classification.

''It gives carte blanche to violate human rights,'' he says. He outlines several scenarios for such disappearances:

* People standing at a bus stop are spotchecked by the police or Army to see if they can produce the correct bus fare.

''If someone cannot show enough money for the bus, he is suspected of waiting to make a contact, and picked up,'' the professor says.

* If a person is seen near a confrontation between guerrillas and the government and is identified as belonging to a different locality, ''he is immediately suspicious, and may be picked up.''

* An armored tank may pull up at a university, and a man wearing a mask looks out of the hatch and points at students for soldiers to pick up and take away. This has happened at least once, the professor says.

* In a denuncias operation, citizens call special telephone numbers to inform on anyone they suspect may be subversive. Callers do not have to identify themselves.

''In 1980 there was one number advertised in the papers,'' the professor says. ''Now there are eight in San Salvador alone.''

Yet observers here point out that not all the disappearances can be linked easily to the government security forces. Some blame right-wing paramilitary groups; others say guerrillas are responsible. But there is rarely evidence about the desaparacidos or eyewitnesses to their disppearances.

One offical at the Salvadoran Human Rights Commission says the chance of a young man or woman disappearing by joining the guerrillas is unlikely.

''The families would say nothing, if that was the case, and go nowhere near the papers,'' he says.

Families go to the newspapers - to El Mundo, La Prensa Grafica, and El Diario del Hoy - with photos because, as one Western diplomat says, ''They have nowhere else to go.''