Jenny Elliot testified at an inquest on Williams' 2010 death, The Sun reported. The body of Williams, 31, was discovered in a padlocked sports bag in his London apartment.
Elliot said she and her husband rented a room to Williams, an agent with the intelligence agency MI6, in their house in Cheltenham because he was working in the town. One night in 2007, at about 1:30 a.m., they heard calls for help from his bedroom.
When they opened the door, they found Williams, dressed only in boxer shorts, tied to the bed. She described Williams as "embarrassed" and said he told her he was "messing around" because he wanted to see if he could free himself.
Witnesses have also said Williams visited Web sites on claustrophilia, the love of being shut up, suggesting he died during a sexual experiment gone wrong.