In case anybody would like to know, I found the activity in: mailbox.log in /opt/zextras/log

Had to search via message ID. Rather helpfully, it reports on which user deleted it if you're running delegated access.

I have tried to sieve through log files to see if there's a relevant log as to what I'm looking for however, I may be being blind, but I cannot find what I'm looking for.

Is there a log which audits user activity such as who deleted an email?

Thanks in advance.