Hi Alex,
share search provides different below options to configure search. You can check - share-config.xml
From below options "context" is set as default option.
none - disable repository wide search option
context - search depending on current context, including repository, all sites, current site (default)
always - always search full repository no matter what the context, disables search context options
<javascript>
<config evaluator="string-compare" condition="Search">
<search>
<!– default minimum length for search terms –>
<min-search-term-length>1</min-search-term-length>
<!– default maximum number of results for a search –>
<max-search-results>250</max-search-results>
<!– Repository search configuration option –>
<!–
The 'repository-search' config element value can be one of:
none - disable repository wide search option
context - search depending on current context, including repository, all sites, current site (default)
always - always search full repository no matter what the context, disables search context options
–>
<repository-search>context</repository-search>
</search>
</javascript>
And I think to limit search specific to any folder, configuration is not available, you need to customize script.