The Diablo 4 Discord listed a website that not only shows you “where” and “when” Helltides will spawn…but the location of the mystery chests! This is a GREAT way to find the loot you’ve been grinding for!
Countdown timer and live map of mystery chest locations when Helltides are active:
https://helltides.com
You can also go to https://d4builds.gg/map/ to study their locations when the other map isn’t live.
If you find a chest using this method let me know in the comments!