grep returns
Binary file test.log matches
For example
echo \"line1 re \\x00\\r\\nline2\\r\\nline3 re\\r\\n\" > test.log # in zsh
You can force grep to look at binary files with:
grep --binary-files=text
You might also want to add -o (--only-matching) so you don't get tons of binary gibberish that will bork your terminal.
-o
--only-matching