What, if any, action should a manager take where they suspect, but have no proof, that an employee's absence was not genuinely for the reason given?

Having gathered as much information as possible about the circumstances, at a private meeting the manager should put the facts and any allegations to the employee, ask questions and seek to clarify any discrepancies or reasonable suspicions. However, the manager should make it clear that what is being put to the employee is not an accusation, but an attempt to make the employee aware of the situation that has arisen and give them the opportunity to present their side of the story so that the truth can be established.