The Pole tested the German marque's DTM car over the winter, but ultimately signed up with Citroen for the second-tier world rally championship in 2014 as he continues his return to full speed in the wake of his near-fatal crash two years ago.
But 28-year-old Kubica, who raced for BMW and Renault in F1 until the end of 2010, has never hidden that - notwithstanding his still injured right arm - his main priority is to get back on the grand prix grid.
Bild said the 2008 Canadian grand prix winner has been testing in Mercedes' F1 driver simulator "for a few weeks".
A figure for a rival F1 team "spotted the Pole at the airport in London", correspondent Frank Schneider wrote.
"His destination was Brackley, Mercedes' headquarters, even though nobody at the Silver Arrows wants to comment," he added.