I'd try this if it were my situation.
1. Check crontab -l for root and other users on master server and media server
2. Check if "bprecover -l" is running - this lists what's on the cold catalog backup tape, it may create an entry in activity monitor as recovery.
3. Remove the tapes from cold catalog configuration, see if it's still mounting.
4. Enable admin log (/usr/openv/netbackup/logs/admin) and VERBOSE 5, to find out what's happening with those mounts
If it's always 7PM (when it happens) then I'd strongly suspect cron, will check out crontab of all the users, on all NetBackup servers, checkout all the scripts in crontab.
Abe