The television spots that have released so far for Avengers: Infinity War have mostly stitched together past footage over new commentary. The latest one, though, features a few scenes that have not appeared before, particularly one that subtly contains a major spoiler for the Soul Stone.
Interestingly enough, the same television spot shared (via MCU Cosmic) on YouTube excludes the scene in question. The version shared on Facebook, however, does not. Whether this was intentional or an accident on the part of Marvel Studios remains to be verified.
When Thanos walks through the portal, his Infinity Gauntlet can be seen glowing with five Infinity Stones. This includes the Power Stone, Space Stone, Reality Stone, Time Stone, and the elusive Soul Stone. The only Infinity Stone remaining is the Mind Stone that is burrowed inside the forehead of Vision
Hence, Avengers: Infinity War will indeed be answering the question of where the Soul Stone has been residing all this time, and the answer will arrive fairly soon in the upcoming movie. In addition, this makes it certain that Thanos will have all five Infinity Stones before heading to Wakanda to obtain the Mind Stone from Vision.
The television spot with the spoiler can be seen here for those interested.
There are a number of fan-crafted theories about where the last remaining Soul Stone currently resides, and in what form, in the Marvel Cinematic Universe. These include some that are a bit far fetched and others that hold some degree of logic.
Avengers: Infinity War will open in theaters worldwide on April 27, 2018, and in China on May 11, 2018. The movie, directed by Joe and Anthony Russo, is the first of a two-part series. The untitled sequel, presently dubbed Avengers 4, is slated to premiere on May 3, 2019.