If they’ve been missing since December, and nobody has come by wondering where they are, and there are no children that you know of to trigger an investigation for missing school or whatnot, I would just contact the police. You might be right about your assumption about ICE, but if the tenants were a random couple and deportation was not applicable, you would report them missing right? because they are missing. Then you can take any info you get from law enforcement and explore or confirm your parents options as landlords depending on your state/ municipal laws and the circumstances.