Yes, you can be terminated for "shift abandonment" even after your shift was over, since (unless you had a written employment contract protecting or guarantying your job) you were an employee at will (employment at will is the law of this country) and employees at will may be terminated at any time, for any reason whatsoever--even incorrect or false ones--or for no reason at all--i.e. if your employer simply wanted to fire you. There is no right to a job unless you had an employment contract, and so you can be terminated whenever the employer wants, and the employer does not need justification to do so.