This is going to put Apple in somewhat precarious legal position, should this technology become reliably sensitive and specific, although it should be easily are resolved.

The issue will be what to do with accurately detected sad mood that might indicate depression, a risk factor for suicide which remains at alarmingly high rates in high income countries. If Siri detects sad mood reliably for a sustained period, should it recommend that the device owner seek help? Should it, as it might do with a detected fall, call for help? Will this be a service that should/will be consented?

In all probability, any calls for assistance, should that be offered, will be by consent. The mere fact that one’s device can detect a reliable marker for self harm or suicide will raise mixed opinions about what should be done with such data, including whether and whom to notify, based on the age group (eg parents of minor children who remain an under-recognised risk group for severe depression).