Bash, compare Array against file and output lines not matched

How about:

IFS=$'\n'
## avoid using CAPS for shell variable names since global env vars are
## capitalized by convention and if yours are also caps, you can have naming collisions
z=( $(/usr/bin/grep -ir --exclude="*_audit.log" ' error' /var/log/*.log | 
      grep "$(date +"%b %_d")" | grep -sv "$non_issue") )

for line in "${z[@]}"; do
   grep -qF "$line" /report.log &&
     printf '%s\n' "$line"
done

The trick is to use -F with grep which:

       -F, --fixed-strings
              Interpret PATTERNS as fixed strings, not regular expressions.

So -F makes grep treat its input as a regular string to find =, ignoring any special characters. Next, the -q means "don't print anything" but the whole thing will only be successful if the line is found. The && means "run the next command only if the previous one was successful" and therefore, if the line is found, then we will print it.

There is the comm command which can show the lines in one file that don't exist in the other file. Assuming that the collected strings from your log files are indeed identical (i.e., will match a whole line in /report.log) to the master file you are checking against, you could do this:

comm -23 <(/usr/bin/grep -ir --exclude="*_audit.log" ' error' /var/log/*.log | 
           grep "$(date +"%b %_d")" | grep -sv "$non_issue" | sort | uniq)   \
         <(sort /regions.log | uniq)

The command by default outputs three pieces of information: lines only in the first file, lines only in the second file, and lines in both. The -23 switch suppresses the output of the last two, leaving you with unique lines in the first file. Note that the command expects sorted files.

If you are unfamiliar with the <( <some-command-pipeline> ) syntax, that is process substitution in bash. You can think of it as bash will pipe the output of <some-command-pipeline> to a temporary file, and then pass that file name to comm. So, this technique saves you from creating temporary files and then running the command on those files.

Here's a simple example of comm that translates to your problem: you want to find lines in the first file (again created via process substitution for convenience) that don't exist in the second. In this case, that would be 1, 2 and 5:

§ comm -23 <(echo 1 2 3 5 5 | tr ' ' '\n') <(echo 3 4 | tr ' ' '\n')
1
2
5