Thank you all for your replies. It looks like my query is not an easy one to answer.
Depending on detecting 'Smartphones' is surely a non starter, in theory.
So you need a 'Smartphone' in the first place, but believe it or not there are still a few people in the World who dont have them or want to buy one just to stay warm in thier own house.
Then of course the 'Smartphone' battery can go flat if you dont religously plug it in each night to charge, so you propose punishing the house occupants for this by turning off the heating.
I'm not trying to take over the world with my solution. My goal is to come up with something that works for my household. If all our smartphone's batteries go flat, I'm OK with freezing for a night because of it.
"It would be really nice if it would know if there's anyone home or not."
Use the forum Google search function in the upper right of this page to search for "occupancy detection" and similar key words. You will probably find many previous project discussions and code using these devices.
I've done that and I found a lot of similar questions but no (feasible) solutions.
PIR should be workable.
work out the simplicity of an onion.
any detection in the outer layer means movement
any detection in an inner layer means presense.
inner lay movement followed by outer layer movement is a potential departure
does not work if one person leaves while others are sleeping.
PIR near exits and PIR closer to the major movement areas would give that two layer value.
Humans breath and CO2 is elevated in the presence of humans. really only applies to areas where humans would be stationary for any length of time.
allow for forgetting your cell phone at work or at home.
I do believe there are other ways to detect the presence of a cell phone.
While it would be really fun to play with all those sensors, I don't think that it would be much fun to have them sitting all around the house permanently.
More and more, I think that my solution (if I manage to come up with any) needs to revolve around the smartphone or BLE beacons.
Anyone has any funky ideas about detecting smartphone presence? (without the Google way - permanent location tracking)