The actual outcome:
As I said, this was an actual case and is used as a "typical" scenario in many university courses and similar concerning ethics, be it as they apply to anthropology or indeed any professional field, when the presence of an ethical code is insufficient to resolve a dilemma.
The anthropologist in question was Marcus Randolph, considered an authority on South American Indian peoples, but in this case was working on a commission to document the declining culture of a particular Pueblo tribe in the USA. In this study he had close communication with the tribe's elders and community leaders; indeed they were joint commissioners of the study, wishing their culture recorded before it disappeared altogether.
Randolph's dilemma concerning the artefact was not one he dismissed lightly, or indeed at all. That it had been stolen was a claim made by the elders, who he trusted. However that it had been stolen by their own people was also undeniable, and that it was only one of many such artefacts which found themselves for sale over the years at Indian trading posts was also true. Their theft was therefore also part of the culture. Randolph set out to exhaustively document all these artefacts' various whereabouts and presented his list to the elders. They agreed that "what's done is done" and that the return of these objects from the various museums and private collections they had ended up in was neither feasible nor helpful any more anyway in arresting the culture's decline. Their desire for the return of the particular artefact in question however was, to them, an exceptional case.
On the basis of their general opinion however Randolph chose not to reveal that he in fact owned the artefact which had sparked off this research. He was honest enough though to include the account, as well as an admission of his ownership, in the eventual publication of his research some years later.
The case is not intended to hold Randolph's decision up as a guideline for ethical behaviour, exemplary or otherwise, since even Randolph had to admit that his dilemma presented him with no good ethical option. He must either betray the trust he had enjoyed from the tribe's elders or betray his professional code. He chose the former, but could as easily have chosen the latter and still have been judged unethical.