There are different signs to recognize that your code is vulnerable, including display issues, functionality issues or poor performance. Specifically, you should think of Magento Code Audit Service in these cases:
Your site is suddenly slow or your sales drop down
Your site is crashed during high traffic and you don't know if the cause is from the code or the server
Some features (in the backend or frontend) do not work properly as they should
You failed to upgrade your Magento version
You wonder if your Magento is the latest version, whether extensions/themes need upgrading, or you might miss any of Magento security patches.
You suspect there is a flaw in your code that creates a potential risk of compromising security
You are skeptical about the code quality and compatibility of themes/extensions installed on your website
Your website does not display as expected in the frontend