I have a few questions regarding these Horadric caches.
1. Do you get one for each bounty you do, or just for completing all five in an act?
2. Do they only come up in bounties period, or are they in other places as well?
3. If I find a cache with one character and open it with another, will the items be geared towards the finder or the opener?
4. If the answer to #3 is indeed the opener, then since the Horadric cache's items are based on the level you were when you found it, will they be that level items for the character class that opens them?
*edit* 5. How does difficulty effect these caches?
1) You get a cache for completing all 5 bounties in one act.
2) Only bounties. There used to be some other cache reward from rifts, but since some players found a way to abuse this (sigh) the "rewards" were moved to the rift guardian, who now drops Bloodshards and has a high chance for Forgotten Souls, amongst other nice stuff.
3) Cache's contents are rolled the second you receive them. Saving them up and opening them on other characters or difficulties has no effect.
4) See 3, if you do bounties with your level 40 toon and open the caches with your level 70 toon you will still get level 40 stuff.
Edit: 5) See #3, or do you mean contents? Torment-only legendaries can only drop from caches if you complete the bounty on Torment, Imperial gems only if you complete the bounty on Master or higher.