THE UNNERVING disappearance of three paintings from three Paris museums within a few hours wasn't the art world's worst recent heist, but the reaction of five small Paris museums was unusually sharp: they announced they would close permanently to individual visitors. When a museum is so worried about the need to preserve and safeguard its paintings that it has to give up the quality that makes it a museum -- the display of those paintings to the public -- then you know something serious is going on. The growing technical agility of art thieves, who sliced a Renoir out of its frame at the Louvre Wednesday without tripping the building's electronic alarm system, is only one element. Such thefts create a destructive nervousness, especially in small museums whose intimacy is their charm and whose small-scale operations won't support the costs of sophisticated security.

Only a tenth of artworks stolen in this way are ever recovered, and many suffer damage, especially when, as in Paris, the theft involves cutting the canvas. But what happens to the rest, and who benefits? Who on Earth would buy a Renoir stolen from the Louvre, especially in this era of instant international headlines when a masterwork is taken? When the Isabella Stewart Gardner Museum in Boston lost a record number of masterpieces this winter, the standard conspiracy theories flew: a mad collector somewhere, a ransom scheme, a ring of Japanese investors. There has been no evidence for these wild scenarios. Constance Lowenthal of the New York-based International Fund for Art Research offers a simpler scenario: rather than looking specifically for "great art," she suggests, thieves may simply grab a painting from a museum as they'd take money from a bank, then sell it for a laughably small percentage of its value to a fencer who knows the goods are stolen but takes the risk. Several transactions later, the initial trail is obscured, and the price is close enough to market range so that a legitimate buyer may simply think he's getting a bargain. Having paid, such a buyer is often reluctant to consider that the work may be illegitimately acquired, which explains why many paintings that are recovered don't reappear for a decade or a generation.

Such a scenario would explain why few art thieves are caught. More ominously, it implies that the theft of a truly world-famous painting is on one level a tactical error; the thief can hardly unload it, and the likelihood of its ever resurfacing is iffy at best. That puts the onus back on museum security, on tighter access and expensive insurance. Museums are on tough times already, and they surely don't need this.