7 – I have a form element that I would like initially to have selected/checked checkboxes for. However, the element already has a default value

I have a form element with checkboxes that uses taxonomy terms via variable_get as default for the form element and stores the result. I don’t want to overwrite the stored value so I just want the form to render checked initially.

  $form('search_block_topic') = array(
    '#type' => 'checkboxes',
    '#options' => $topic_options,
    '#default_value' => variable_get('search_topic_options', $topic_options)

I tried
'#return_value' => "checked",
which doesn’t work.