Some say Putin's affair with spy-turned-lingerie model Anna Chapman, which has been strongly denied, is the reason the former Aeroflot-hostess is now rarely seen out in public.
Others say he is still seeing former Olympic gold medallist Alina Kabayeva, claims also heavily rebutted, but who he was alleged to have fathered a child with.
A third theory is his 54-year-old love is 'locked away' in a 1million pound state-built guest house in the grounds of the ancient Yelizarov monastery outside Pskov - close to Estonia's border.
Locals say they have 'proof' she has been there - but all official enquiries to the authorities are met with a steely silence.
A final hunch is Lyudmila became angry when, in 2008, he stepped down from the top job but then took up the post of Prime Minister.
She is rumoured to have moved back to his native St Petersburg while he remained in the capital Moscow, the Daily Mail reported.
Sociologist Olga Kryshtranovkaya said Putin was merely copying Soviet leaders who hid their wives from view.
The one exception, she said, was Mikhail Gorbachev whose wife Raisa played a higher-profile role during the 1980s - but that led to criticism from ordinary Russians.
"For Russia, the status of First Lady is different than in Western countries. It's our mentality about the role of women, unfortunately," she told The Times.
"Our First Lady has practically always been a housewife who looked after the children and didn't invite herself into politics. The first change was with Gorbachev and it was a very unsuccessful example from the Russian point of view," she stated.
Putin, 59, will be inaugurated on May 7, then fly to the U.S. on May 18 for the annual G8 summit of world leaders.
Previously, they were often pictured together on diplomatic trips abroad, but n the last two years they have been seen together just twice.
In October 2010 the couple, married in 1983, tried to quell rumours they had divorced by posing for pictures as they jointly answered questions for the national census.
Then, on March 4, she was spotted leaving a polling station after casting her vote. Little is also known about the couple's two daughters Maria, 26, and Yekaterina, 25.
Russia's media has never published photographs of their daughters and no family portraits have ever been released.