I have had some basic sensors at my mom's house for 4 (or 5) years - bluetooth scan to see if her phone is around and on (she refuses to use a "smart" phone wants a simple clamshell), CCTV, etc.
The problem is that there is a very high rate of false positives and bitrot. If you do not tend to it regularly you will find another glazed china mouse collecting dust right in front of the motion sensor or the cctv camera exactly when you need them to work.
In any case, the setup misses the most reliable indicator that your fav pensioner is all right. Tracking that is trivial. All you need is LIRC with the correct files for her remote controls. If a 70 years old has not touched the TV for more than 3 hours, that usually means trouble.