The Term Database (TD) search in Browser Workbench is not case sensitive. If you have a TD match that matches the case of the term in your source segment and one that does not, both will display in the TD lookup window. Here is an example:
However, keep in mind that only terms that are in active/approved status are displayed. "Proposed", "Inactive" or "Rejected" term will not be displayed, as explained in this article.