How can I edit the search form?
It's displayed using the Context module.  I found it like this:


Go to admin/build/context and select the global context

Click the Blocks reaction
Under the Space tools region, Atrium: Search form block is being shown



I then searched for the name Atrium: Search form, and found that the block is defined in the atrium_features module, in functions atrium_block and atrium_search_form of file atrium/atrium.module.

You could modify the search form using a hook_form_FORM_ID_alter hook, such as hook_form_atrium_search_form_alter.