What should a person do who was fired from his job in 1996 for refusing to relay news about colleagues, and who took some things from the company, after all these years have passed?

The thief must repent to Allah and return the stolen goods to their owner or seek their forgiveness, or compensate their heirs if the owner is deceased, based on the saying of the Prophet (peace be upon him): "Whoever has wronged his brother concerning his honor or anything else, let him seek his forgiveness today before there is no dinar and no dirham." If he fears corruption, he should send the compensation to him in an appropriate manner. If he is unable to return it, it becomes a debt upon him, and he must record it in his will. If he does not know the whereabouts of the rightful owner, he should give its value in charity on their behalf.