git annex whereused would report where in the git repository a key is used, as a complement to git-annex unused.

Use cases include users not getting confused about why git-annex unused says a key is used.

Also, it could scan through history to find where a key was used. git-annex unused outputs a suggestion to use a rather hairy git log -S command to do that currently.

If it does both these things, it could explain why git-annex unused considers a key used despite a previous git rev referring to it. Eg:

# git annex whereused SHA1--foo
checking index... unused
checking branches... unused
checking tags... unused
checking history... last used in master^40:somefile
checking reflog... last used in HEAD@{30}:somefile