Problem
Some files were not returned in the results despite containing my search term. They were returned it only when they were opened in a tab.
Cause
It appears it was caused by ST4 taking into account the files in my .gitignore as a source to exclude from search. Which was unexpected.
Solution
When search function is opened, click on “Use gitignore” button on the left side. Caution: does not change state. Meaning it looks the same activated or deactivated, so its really easy to miss it.