HR management question

I have an employee that went to a work related conference that ended 5 hrs into the work day. The employee went home instead of driving an hour to make it into the office. The employee may or may not have worked from home. I am considering writing the employee up for not working the full eight hrs that was filled out on the time card? I have a suspicion this employee has done this same type of thing one time prior.
What action should I take if any?