Warning — Thor: Ragnarok spoilers below!
In Thor: Ragnarok, one of the big headlines by the end of the movie is that Asgard, Thor and Loki's home planet, has been destroyed by villains Hela and Surtur, so Thor has to take a giant craft full of his people to a new place. Meanwhile, throughout the movie, Loki goes from being chill to mischievous and back again, in sort of a roller coaster of betrayal. It certainly seems that Loki is back on the good side when he helps Thor get the Asgardians off of Asgard, but there's an open question at the end of the film that implicates Loki. That question is: what happens to the Tesseract Odin was storing in Asgard, the magical object that's popped up throughout Avengers movies?
While it's not revealed in the end credits, it's entirely obvious where that thing is: with Loki. Loki has been trying to get his grubby hands on the Tesseract since the first Thor movie, and he succeeds in stealing it in the first Avengers installment.
Toward the end of Raganarok, there's a pointed moment where Loki sees the cube while walking through Odin's treasure room, which would have been his chance to snag it. It's Thor who had brought it back to Asgard (along with Loki) to keep it safe, so there is a chance that Thor remembers to grab the cube before Asgard goes boom, but it doesn't seem to be top of mind for him. By not showing what happens to the infinity stone — and knowing it's too valuable to be destroyed — we can only assume that the prankster Loki has it hidden away somewhere. Hey, Loki can only be good for so long.
We should definitely find out the Tesseract's fate when Avengers: Infinity Wars comes out next May, since the villain Thanos is trying to get all the infinity stones — and that may be exactly why his ship pops up in the Ragnarok end credits.