Based on observation/inference from my mailbox.logs, aname (seconduser) is a user who is accessing a shared resource of name (firstuser).
You might be able to get more information on what's being accessed by changing the loglevel for the user:
zmprov addAccountLogger user@domain.com zimbra.soap debug
See
Log Files - Zimbra :: Wiki and note that there are other categories of logging besides zimbra.soap, but that's probably the one you want.
You could also do "view mail" from the Admin GUI, on the source account, to see what they have shared. Or you could use zmmailbox:
zmmailbox -z -m user@domain.com gaf
then iteratively step through the folders:
zmmailbox -z -m user@domain.com gfg /path/to/folder
This can be tedious, but it shouldn't be too hard to write a shell script to automate the process.