The 'On The Road' actress, who split from the British actor in May after four years together, spent three hours inside his house before she was spotted following his jeep out of his gated community in Los Angeles in her pickup truck and driving over to her own house.
An insider told UsMagazine.com that the former couple drove off in different directions when they realised they had been identified by photographers and Kristen went back to her property alone.
The duo, who met on the 'Twilight Saga' trilogy, called off their relationship after they struggled to get past the 23-year-old actress's "momentary indiscretion" with her 'Snow White and the Huntsman' director Rupert Sanders in July 2012.
Robert and Kristen were last rumoured to have spent time together in August, when a source predicted they would reunite after having a "very intense conversation" at Kristen's house.
The insider said at the time: "They will probably pick up where they left off. Rob is obsessed with her."
The 27-year-old heartthrob has recently been linked to Sean Penn's daughter, Dylan Penn, and also enjoyed a brief romance with Riley Keough earlier this year.
Kristen was rumoured to be dating her 'Camp X-Ray' co-star Lane Garrison in August.