If they ask for the money, yes, you must return it. The law is very clear that a mistake does NOT allow you to keep money to whch you are not otherwise entitled. Just as if the employer made a mistake and paid you too little or missed some paychecks for you, they'd have to correct that mistake and pay you what you are owed once the error is discovered, so, too, if you are kept on payroll by mistake when you are no longer working, you have to return the money. If you do not, they can (and presumably will) sue you and win.