20 Unanswered Questions From 2017's Comic Book Movies
4. Where Was Sif? - Thor: Ragnarok
The Question: Thor's pal Sif (Jamie Alexander) was conspicuously absent throughout Thor: Ragnarok, so where exactly was she while Asgard was being sacked by Hela and the Warriors Three were brutally murdered?
The Likely Answer: Alexander was unable to appear in Ragnarok due to scheduling conflicts, but if she was due to suffer a fate similar to the Warriors, it was probably for the best that she was unavailable.
It's likely Sif will make another appearance in the MCU at some point, so she most certainly didn't die off-screen, and it'll most likely be explained - if it is at all - as her tackling a mission in the far flung reaches of...anywhere that isn't Asgard.
It won't be terribly convincing that she wouldn't return to help fight Hela, though, so it'll probably be glossed over pretty quickly if she does indeed show up again.