When Christie’s holds its antiquities sale today, one object whose photograph appears in the catalog will not be present: a 4,000-year-old Egyptian alabaster offering vessel pulled from the auction on Monday, after a Metropolitan Museum curator recognized the object and tipped off Christie’s that it may have been stolen from a storage facility in Egypt. In an atmosphere in which the antiquities trade is already under suspicion because of rising concern about cultural patrimony and looted art, the case at Christie’s highlights just how difficult it is to determine an object’s provenance and authenticity.

The Met curator recognized the offering vessel in the catalog because he had taken part in excavating it in Egypt in 1979. And he knew the object had resided at a facility in Egypt. “I don’t think it was a matter of [his] saying it was stolen,” the Met’s spokesman, Harold Holzer, said. “It was a matter of saying, ‘The last I heard, the object was stored at this particular facility, and I thought I’d let you know.’ And I think he was not surprised by the course that it then took.”

Christie’s responded quickly. “[The Met] sent us a photo of it and it matched, ” the head of the antiquities department, G. Max Bernheimer, said. “And this proved to us beyond a shadow of a doubt that this piece had to have been taken out of Egypt illegally.” Mr. Bernheimer said that Christie’s will hand the vessel over to the U.S. Department of Homeland Security.

The case raises questions about how thoroughly auction houses and dealers can research the provenance of the objects they trade in.Mr.Bernheimer said that Christie’s checks all the objects that come in against the Art Loss Register. (The register is a database of missing art objects, maintained by a for-profit company headquartered in London, in which the major auction houses, including Christie’s, are shareholders.) Of course, the Art Loss Register cannot list stolen objects that have not been reported or objects excavated illegally, which have no official record at all.

Mr. Bernheimer said that Christie’s also does some research to check the information provided by the vendor. “If they tell us a piece was published in such and such a journal or in such and such an exhibition catalog, of course we follow it up. We do everything that we can do, but there’s only so much that you can do.” Beyond that, the auction house has to rely on the veracity of the vendor, who has to state in the Christie’s contract that he has clear title to the object.

In the case of the vessel, the catalog gives its provenance as an “Israeli Private Collection, acquired prior to 1975.” That the object was excavated after that date means that, somewhere along the line, a provenance was fabricated.”I don’t know where the lie came into effect,” Mr. Bernheimer said. “The vendor told us that they bought it from a collection and that the collector from whom they bought it said that it had been acquired prior to 1975, so obviously that’s where the problem is.”

But the vague note of provenance in the catalog points at part of the problem in the trade in general, said an archaeologist and the editor of the International Journal of Cultural Property, Alexander A. Bauer. Auction houses and dealers willingly protect the privacy of sellers by providing them anonymity – meaning that a history of ownership often can’t be produced as it can for any other commodity.

“The art trade, including auctions, is historically shrouded in secrecy,” Mr. Bauer said. “When provenance is declared, it’s ‘from a very well-known established collection’ – but they don’t actually say what that collection is.”

This secrecy, he said, allows dishonesty to slip through – not just stolen or looted objects, but out-and-out fakes. “I can understand why [they want to be anonymous], if somebody’s trying to sell off their estate and they don’t want people to know they’re in financial trouble,” he said. “But it does a disservice to those who desire that the trade be considered legal and looked on in a good way.”

The executive director of the International Foundation for Art Research, Sharon Flescher, said the major auction houses try to do as much research as they can, given the number of objects and the limited time. “But in the end, the antiquities field is a complicated and problematic one,” she said, “and there’s always an element of caveat emptor.”

The president of the Archaeological Institute of America, Jane Waldbaum, had a more critical take. “Could they have found that out on their own? I think most people would presume that if you go to an auction at Christie’s that they’ve done the work for you.You can take it on faith that it’s okay. This calls all that into question.”

In this case, Christie’s misfortune may have been the Met’s boon. The Met itself has been under a shadow in the recent years about possessing antiquities to which Italy has laid claim. Earlier this year, the Met signed an agreement with the Italian government, in which the museum agreed to return 21 disputed objects in exchange for loans of others.Of the Met’s alert to Christie’s this week, Mr. Bauer said, “Certainly the Met’s doing this is an indication of their changing attitudes, and their desire to play a more positive role with regard to the intentional trafficking.”