Oh dear....what a nightmare you went through - and thank you for the heads-up
I personally tend NOT to leave Harry with my parents (also in their 80's) for the reasons you stated. Plus my dad (especially) tends to open the front door and gaze out...generally trying to see what the neighbours are up to!!!
We have 2 people that we leave Harry with when we go on holiday (if we can't take him). VERY vetted by ourselves, and we're happy in every way.
For those people, we have the appropriate tags made and attached to his collar - THEIR phone number, address etc.
We also inform our vet, just in case...
Sally + Harry x