The Formula Three champion will move up to F1 next year with the British team and has been undergoing a secret worldwide testing schedule, but deputy team principal Claire Williams won't open up too much about the programme.
"I can't go into too much detail," she told Auto Motor und Sport. "But he drove on a handful of European and non-European tracks. In the end, he will have driven about 8,000 kilometres."
"It's to make him familiar with the job, working with the engineers, all the processes and the way the race weekend runs, including pit stops and starts.
"His preparation is unprecedented at a time when basically no testing is allowed," Claire admitted.
The programme, almost certainly funded by Stroll's billionaire father, was kept secret in order to shelter the teenager from any unwanted pressure.
"We wanted to take the pressure off him. And he shouldn't be distracted from his work," said Williams.
"A lot of people are interested in him and his story, not only because of his talent, but also because he has a well known father."
Bahrain International Circuit - Winter testing