It is possible that it has been turned off. The vardefs.php file contains a property that turns auditing on or off for that particular module. Check your modules/Leads/vardefs.php file and make sure that the "audited" property is set to true as shown below.
Code:
$dictionary['Lead'] = array('table' => 'leads','audited'=>true, 'unified_search' => true,'duplicate_merge'=>true, This functionality exists in Sugar OS.
See two screen captures below. The first shows you where the link would be displayed. The second is what the popup looks like.
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