Who are the cunning masterminds who rob the great museums? Where are the half-crazed millionaires who hang stolen Rembrandts in their bathrooms? Is the Mafia becoming involved in art crime?

The true stories the policemen have been telling here for days -- of priceless paintings stolen, rope ladders through skylights, alarms disconnected, display cases smashed, and museum guards shot down -- would be oddly disappointing for fans of fiction. For though the objects stolen from the great museums -- the Gauguins and Picassos, the jeweled swords and bronzes -- are the opposite of common, the same cannot be said of the crooks who pull the crimes.

Art theft is increasing. But the more than 60 curators, art police and collectors, assembled here for a four-day international Symposium on Art Security, apparently agree that art thieves on the whole are garden variety criminals -- burglars, junkies, sneak thieves -- people much like those who steal typewriters, autos, TV sets and dogs.

"Let me dispel some myths," said Gilbert Raguideau, the suave Gallic officer who heads the Central Office for the Repression of the Theft of Art Objects for the Government of France. "There is no mastermind, no international art Mafia. We all have heard the legend of the mad, rich connoisseur who buys stolen masterworks. He does not exist."

Donald L. Mason, the former FBI-man who helped organize the university of Delaware's symposium, agrees with Raguideau. "We are dealing here with a specialized kind of property -- not a specialized kind of crime."

Beautiful old paintings retain a special glamour, even when they're stolen by the dullest sorts of thieves. There is something fascinating about art theft stories. The academicians who organized the Delaware symposium understand that, so, too, do the policemen who, with relish, tell their tales of art crime.

Mason tells the tale of the painting by El Greco that was stolen from Madrid in 1936 and was not recovered until 1971. It was Raguideau who hired a small airplane in which he tailed the Picasso thieves -- they'd stolen 119 paintings -- who were racing to the border on a highway far below. "Not more than two weeks ago, only a few yards from the Opera in Paris, we recovered an Hieronymus Bosch that had been stolen, and then held for ransom, by disgruntled Basques."

Police Sgt. John Killoran of Worcester, Mass., spoke of the gang of thieves who -- at gunpoint -- robbed his town's art museum. They took a Rembrandt, a Picasso, and a pair of Gauguins with a total value of $2.5 million -- and they shot a guard. Lt. Ralph Atkins of St. Louis discussed the criminals with sledge hammers who not once, but twice, smashed their way into the St. Louis Art Museum; the bronzes they escaped with were by Rodin and Remington.

Yet such "horror stories," thrilling as they are, in many ways mislead. The 119 Picassos, the Bosch, the Gauguins and the Rembrandts -- like most other precious paintings taken from museums -- eventually were recovered. All these cases were resolved through traditional police work. The police did what they always do when precious things are stolen -- they went into the streets, contacted informers, and followed up their leads.

Thefts from great museums have been grabbing headlines of late. A 2500-year-old Grecian marble head was taken just last weekend from the Metropolitan Museum of Art. Three oils by Cezanne, worth perhaps $3 million, vanished last December from a storeroom in the Art Institute of Chicago and are still missing.

But masterwork theft may not be the major problem. The recovery rate for stolen Rembrandts and Cezannes is vastly more encouraging than that for lesser works of art.

Police estimate that nomore than 5 percent of all the art that's stolen is recovered by its owners. But a study just conducted for the International Foundation for Art Research notes that of the 10 "major North American museum thefts" that occurred between 1972 and 1977, eight have since been solved.

Is there a major boom in the theft of works of art? No one knows for sure, statistics are inadequate: The answer seems to be both yes and no.

There is no doubt at all that there is a boom in art -- more people than ever are making it, studying it, seeing it in galleries and museums, and stealing it as well.

Yet despite the "art theft boom," insurance rates for works of art are lower now than ever.

A picture worth $1,000 may be fully covered for a tiny premium of $1 a year. Compare the rate for cars.

"By consensus of the underwriters interviewed, fine arts insurance has been very profitable," writes Bonnie Burnham in a new report on art theft. "The casualties experienced, when measured against the enormous values insured, have been extremely small." But insurance agents do occasionally get burned. The policy that covers the $3 million Cezannes still missing in Chicago was sold to the Art Institute by a Washington insurance man, Huntington Block.

"Insuring art is safer than insuring luggage, say, or jewels," observes Andrew J. McDonough, a Boston insurance salesman. "Art collectors love their works. On the whole they're trustworthy. They do not often cheat."

"But criminals," McDonough adds, "know there is liquidity in lesser works of art, old coins, for example, or prints, or antique silver. Such things are hard to trace. If you had to smuggle money into Europe, you could slip into your pocket $500,000 in untraceable, rare stamps."

Though the governments of Italy, France, and other European nations have organized art police squads, there is, in the United States, only one policeman -- New York's Robert Volpe -- who is assigned full-time to the field of art crime.

"Art security," on the other hand, has become a booming business. Mason retired from the FBI to become an art security consultant. And he has just published a book on his subject. While policemen spoke of cases in the lecture rooms at Delaware, sophisticated wares -- passive body heat detectors, pulsed infrared photoelectric intrusion detection systems, locks and keys and ultrasonic detectors -- were being sold outside.

Security systems designed for other industries are now being widely used to protect works of art. Ralph Ward, a McLean, Va., security expert, showed slides of TV consoles, concealed door hinges, fire-proof safes, polygraphs, and guards.

Ward said no security system is foolproof, that fire is a greater danger than thievery, that skylights can be protected "by installing detection systems upside down," and that "inside jobs" are common. "You may not like to hear this, but 85 percent of everybody's losses go out the door marked 'employes'."