This could be related to a known bug in 3.30.1, fixed in 3.31.0 (ERA-1781).
If an audit was still unfinished and its planned date had already passed, editing the parent control could recalculate the schedule and remove that audit. Completed audits and future scheduled audits were not affected by this specific issue.
Were the contact changes made on 22 October, while you were still on 3.30.1? And was the missing audit still unfinished at that point?
In the activity log, the Foreign Key identifies the audit, although I agree it’s not very easy to read, since you need to find the audit’s ID and match it with the log entry 
. Showing the audit’s name and related control would make this much clearer.
In your case, the email URL has audit ID 1345, and your screenshot shows a deletion entry for that same ID on 22 October, so that entry does refer to the missing audit.