The rapper turned up at the "Hello" star's house alongside a hefty entourage, with a source close to Adele telling Britain's The Sun newspaper, "It was all a bit cloak and dagger. He arrived in the middle of the night and didn't leave until around 2am."
"He arrived in huge, blacked-out cars, with plenty of muscle surrounding him."
While the unexpected friendship between Adele and Brown is likely to come as a surprise to many of their fans, they've actually been pals since first meeting at the 2013 Grammy Awards.
After they were pictured chatting at the awards, Chris tweeted, "Pleasure to meet you, Adele. I heard your words and that's all that matters. Thank you for speaking truth."
He's also a close friend of British grime star Skepta, who was previously rumoured to be in a romance with Adele.
Meanwhile, it was also something of a shock to see Brown in London, given that he was banned from visiting the U.K. after he was found guilty of assaulting then-girlfriend Rihanna four years ago.
Back in 2013 when a picture of Chris Brown and Adele at the Grammys first made its way out, it was rumored there were issues between them but Adele was quick to shut down the reports. "Chris Brown and I were complimenting each other in that photo actually!" so she wrote on Twitter to clear up the air back then.