Where does Strongbox store its local working copy/cache (for offline access)?

Strongbox's offline/working copy (or cache) is stored under:

<Shared App Group Directory>/sync-manager/local

The  [1] is something that Apple provides to Apps and isn't something that can be viewed by ordinary users on iOS, but it is visible on macOS. The API that provides this directory is:

containerURL(forSecurityApplicationGroupIdentifier:) see

The group name parameter is:

iOS: group.strongbox.mcguill
macOS: group.strongbox.mac.mcguill

On macOS, these files are likely to be found at the path below, though this is system dependent and open to change by Apple so you should not under any circumstances interfere with files in this directory.

~/Library/Group\ Containers/group.strongbox.mac*

[1] https://developer.apple.com/documentation/foundation/filemanager/1412643-containerurl