Interestingly, Katie's ex-husband Tom Cruise was also nearby.
"Jamie wrapped filming on 'Robin Hood' and Katie flew in to Paris to meet him for a few days," a source told E! News on May 12. "Tom Cruise was filming 'Mission Impossible 6' only a few blocks away, but Katie and Jamie stayed inside their hotel…and didn't leave."
The new take on "Robin Hood," which stars Jamie, was filmed in Croatia and Hungary.
E's source said the duo was scheduled to head back to the United States earlier this week, but there was a problem with the plane, so they stayed longer.
"Jamie had a big smile on his face as they returned to their hotel and snuck in through a private entrance," the source said. "They were very discreet and made sure to always enter and exit the hotel and the car separately."
Another source said Jamie and Katie flew out of Paris privately on May 11.
The duo has been the subject of romance rumors since 2013, although they have never confirmed any kind of romance — in fact, in the past, Jamie has implied that they are "just friends."
The romance, though, is "not a secret among their friends," E!'s source said.
In April, the two were photographed having dinner.
Jamie and Katie were first spotted dancing together in August 2013 at a party in New York. Since then, they've been rumored to have taken part in a ton of secret rendezvous. In 2014, the New York Post said she flew private (as to not draw attention) to California to stay with Jamie at his home around the Grammys. They also reportedly hooked up during Super Bowl weekend in 2014 but appeared separately at the game. A New Year's meet-up in Miami was also rumored.
Fast forward to 2015, where it was revealed that Katie says those three magic words to him — you know them: "I love you." She also reportedly wore a disguise to meet up with him in secret in Los Angeles, Us Weekly reported.
"Oh come on, you guys have been trying to get that to stick for three years," Jamie responded to a paparazzo's question about how things were going with Katie in 2015, saying the two were "just friends."
In June 2016, Jamie's pal, former "Real Housewives of Atlanta" star Claudia Jordan, seemed to confirm the relationship on the Allegedly podcast.
"[He's a] good friend of mine. He is very happy with her," Claudia said when asked about the status of Jamie's romance with Katie. "I like that he seems very happy."
However, less than 24 hours after making the comments, Claudia backtracked and said she "misspoke."
"I have no knowledge of Jamie with Katie at all," she told Entertainment Tonight. "I've never seen them together, he's never told me he's dating her."
In September 2016, another fascinating report emerged on Radar's website claiming that the reason Katie and Jamie had kept things so quiet is because there's a clause in Katie and Tom Cruise's divorce decree stating that she can't publicly date anyone until five years after the end of the marriage. Their divorce was finalized on Aug. 21, 2012.