On a CentOS system I have to hand the, what to do when various disk space limitations are met is structured ...
space_left = 75
space_left_action = SYSLOG
admin_space_left = 74
admin_space_left_action = SUSPEND
This causes this message to be issued to syslog when free space on the partition housing the audit log goes below 75MB
May 21 08:53:01 c6test auditd: Audit daemon is low on disk space for logging
Similarly when space goes below 74MB this message is issued to syslog
May 21 08:54:01 c6test auditd: Audit daemon is suspending logging due to low disk space.
So to answer your question, it writes a message to syslog which then finds it's way into your system logs so monit can be configured to look for it.
What the actual message is and which log file it gets written to is likely to be dependent upon OS and/or distro.