Missing treatment
If a patient does not come to treatment, the goal is to follow up with them to find out why. If this is a reoccurring occurrence you can devise a plan to find out the best course of action that can include -
calls to the patient, family, HCP
referrals to Adult Protective Services, Mobile Crisis, police (wellness check)
letters in writing to the patient advising concerns and Medical Director's recommended safety plan.
If the patient is missing for over 30 days time then you can consider removing them from the census, but if you have been in touch with the patient and know where they are you are not able to discharge.
If they are missing for over thirty days time and you have no idea where they are, despite all efforts -- only then can they be classified as lost.