Netbackup Policy Change

1/ If EMM auditing is turned on, its logged in there  - run nbauditreport command

2/ Run bperror command - there no file that contains the NBU errors daily, there are many logs that you could look in to build up a picture - I think bperror is the best option.

3/  If the policy is deleted, it's gone.

You could recover it from the catalog backup, by chasing, in the BAR gui the policy type to nbu_catalog.  this allows the catalog backups to be browsed like normal backups, and the policies are found in /usr/openv/netbackup/db/class  - the individual polcies could be restored from there.

Auditing is enabled by default on NBU 7.6 master server. 

See: About NetBackup auditing 
http://www.symantec.com/docs/HOWTO105855 

Have a look at all the links in above HOWTO. 
Such as: 

See Viewing the audit report

See Configuring auditing on a NetBackup master server

See Viewing the current audit settings

See Audit alert notification for audit failures