Now, just a few hours later, fans of the show reckon they might have figured out who it is.
Taking to social media to share their theories, viewers noted that the mysterious figure - who seems to be sporting patterned socks and looking out over the ocean from a hammock in the new teaser photo – strongly resembles The Royal Family star Ralf Little.
Little previously appeared in the Caribbean-set show in the sixth episode of Death in Paradise's second season.
He starred as Will Teague, a holidaymaker who arouses the suspicions of Detective Inspector Richard Poole (Ben Miller) after an English tourist is found dead in his luxury villa.
Might he be playing the same character, or will he be playing someone completely different if he has come aboard?
If the replacement turns out not to be Little, other fan suggestions have included Hugh Laurie, Doctor Who star Christopher Eccleston, Pointless presenter Alexander Armstrong, Gavin & Stacey's Mathew Horne – who appeared in the show back in 2013 – and even... James Bond himself Daniel Craig.
O'Hanlon announced last week that he was saying goodbye to his beloved character DI Mooney.
"I've spent the last four summers in Guadeloupe playing Mooney, solving over 20 murder cases in my time there!" he said.
"It's been a hugely rewarding and unforgettable experience, working with incredibly talented actors, directors, and crew while discovering a magical part of the world."
Death in Paradise returns to BBC One in January 2020.
Digital Spy is launching a newsletter – sign up to get it sent straight to your inbox.
You Might Also Like