It’s In The Quantum Realm
It’s quite commonly thought that the Soul Stone might not even appear in Infinity War, at all. It’s possible that Thanos ends the film with five out of six Stones and the final one eventually turns up in Avengers 4. If that’s true, maybe that’s because the Soul Stone will only be found in the interim between the movies. Say, for instance, in Ant-Man and the Wasp?
Handily, the first Ant-Man already introduced somewhere that would be perfect to hide something you didn’t want to ever be found – the Quantum Realm, the other dimension that can be accessed if you shrink to a sub-atomic level. We know that the sequel will revisit the place as Michelle Pfeiffer’s Janet Van Dyne will be rescued from there.
The Quantum Realm was also glimpsed in Doctor Strange, revealing that sorcerers can access it. Perhaps the Ancient One hid the Soul Stone there hundreds of years ago… but Scott Lang accidentally brings it back with him?
Likelihood: It would be a clever move by Marvel to make use of the fact that Ant-Man and the Wasp is sandwiched in between the two Avengers movies. And we do know that screenwriters Stephen McFeely and Christopher Markus had to ask director Peyton Reed and his team to do certain things to set up Avengers 4.