Always display extra fields for all categories in search/filter module

Suppose you have custom fields assigned to all categories you may want to always display in the Filters/Search Module. In this case, users' won't need to choose any category to reveal the custom fields applied to all categories.

Then go to the extra field that you want to be assigned to all categories (you can assign a field to all categories by opening it in the back end [Edit tab], scrolling down and clicking the "Add to all categories" link.

Once the field is assigned to all categories, open the "Search" tab in the very same extra field and select "Show in search on start" to "yes" and save the extra field.

Now, this extra field will be visible when no category is selected in the module.