A flow of breathable gas is delivered to a subject through a patient interface assembly. An enhanced technique for determining whether the airway of subject is disengaged from the patient interface assembly. A model that provides predicted values of a dynamic property of the flow of breathable gas is fit to measured values of the dynamic property. The model and/or the fit of the predicted values to the measured values are then analyzed to determine whether or not the airway of the subject is disengaged from the patient interface assembly. This provide enhanced determination of these conditions even in implementations in which a restrictive patient interface assembly is used.
