The military identified the woman as Noreen Leghari, a medical student who grew up in the southern city of Hyderabad.
After a series of attacks in Pakistan left more than 130 people dead in February, Islamabad shut down its border crossings with Afghanistan.
The bike race was one of many events organised to promote female participation in public events and to fight restrictions faced by women in public places.
In 2015, the World Bank estimated Pakistan’s population at 188.9 million but the country still uses the official 1998 figure of 134.7 million.
Relatives were unable to shed any light on who had been holding the men or why.
In recent days, the missing Pakistani activists have been accused of blasphemy in online posts and by at least three television commentators.
A Dawn editorial on Tuesday implicated the state in the disappearance of the missing activists, including poet and rights activist Salman Haider.
Even as the Nawaz Sharif government tries to honour Nobel laureate Abdus Salam, an Ahmedi, by renaming the physics department of Quaid-e-Azam University in Islamabad after him, persecution of the community continues.
How internal discord in the Muttahida Qaumi Movement could spell doom for Pakistan’s maximum city.