Bakhmina, 39, former deputy head of the legal department of the now defunct oil company's Moscow unit, has been serving a six and a half year term on embezzlement and tax evasion charges since October 2006. Bakhmina, who has two children, is pregnant and is due to give birth in December.
Kommersant daily reported on Monday that Bakhmina had left a clinic in the Russian Volga republic of Mordovia, where she was transferred several days ago, and that her whereabouts were unknown.
"After reports that Svetlana is not in a hospital in Mordovia, we contacted the local penitentiary authorities, who told us that she is in a clinic near Moscow," Roman Golovkin said. "At the moment, I do not know where exactly she is."
Kommersant cited Bakhmina's defense team as saying that her transfer from a clinic in Mordovia to an apparently better maternity hospital could be part of a deal with penitentiary authorities, who could have promised her better hospital conditions and parole in the future in exchange for a pledge not to seek a pardon.
Bakhmina, who had maintained her innocence since being arrested in 2004, pleaded guilty on October 22 and requested to be pardoned.
Local penitentiary authorities apparently do not want to get the president, who decides on pardons, involved in the affair, and they fear Bakhmina's possible health problems could damage their reputation, the paper said, citing a defense team member.
In September, a court in Mordovia turned down her request for parole despite her pregnancy and testimony to her good conduct in prison. It was the second such rejection by the court.
The business newspaper Vedomosti, however, said on Monday that lawyer Golovkin had lodged another plea for pardon on Bakhmina's behalf on Friday.
Golovkin was quoted by the paper as saying that with Bakhmina "preoccupied with her health at the moment," he had decided to file a plea on her behalf.
Bakhmina's mother was reported to have approached the presidential administration on Friday, asking for her daughter to be pardoned. "My daughter has made the confessions... All she wants is to be with her family and to raise her children."
Administration officials have refused to comment on the mother's request. The Federal Penitentiary Service, quoted by Vedomosti, said Bakhmina alone had the right to file a pardon petition.
Some media reports said she withdrew her pardon request on October 24.
Both papers cited Bakhmina's husband on Monday as saying that he last talked to her on the telephone about a week ago, when she said that her state of health and the hospital conditions were satisfactory.
"Svetlana asked me not to look for her, not to visit or bother her with phone calls," Mikhail Zhuravlyov was quoted as saying by the papers.
Legal proceedings launched against Yukos in 2003, seen by many as politically motivated, resulted in the conviction of Bakhmina and other executives and shareholders of what was Russia's largest oil producer, including founder and CEO Mikhail Khodorkovsky.