https://www.gamespot.com/articles/theres-a-rainbow-six-patriots-easter-egg-in-the-ne/1100-6460505/

Following the Splinter Cell missions with Sam Fisher, Ghost Recon: Wildlands is crossing over with yet another Tom Clancy franchise: Rainbow Six. We already spent several hours with the update--which is available now on all platforms--and toward the end of the content we discovered a tongue-in-cheek reference to Rainbow Six: Patriots, one of Ubisoft's notorious cancelled projects. Spoilers follow for Ghost Recon: Wildlands' new update.

In addition to a new permadeath difficulty and updates to its PvP mode, Wildlands' newest free DLC update brings several characters from Rainbow Six Siege to open-world Bolivia. After discovering the scene of a firefight in which several Cartel members didn't make it out alive, you're introduced to Valkyrie, a fan-favorite character from Siege. She informs you that a small contingent from Team Rainbow is in Bolivia searching for one of their lost members: the Brazilian operator Caveira.