Mentioned and discussed elsewhere, but posting here for completeness…
Vehicle Presence Detection for OpenHAB. Fairly self-explanatory, determines if your car is home or away, allowing you to perform automated tasks in rules based on that state.
2 Likes
I would use the Deep Sleep from the Arduino to protect the battery.
Just throwing in a completely different, stationary approach. There is software that recognizes license plates of cars and other vehicles. You could use this to detect whether a specific vehicle is in a specific location or do it for many vehicles at once. Combined with a camera with night vision capabilities (read: IR LED) this could be a fun project 
https://github.com/openalpr/openalpr (ready to use)
https://cvisiondemy.com/license-plate-detection-with-opencv-and-python/ (deep dive)
Living in a rented flat I have no way of doing this but please keep me in the loop if you try this. I find this super interesting! 
Thanks for the replies, however…
Regarding deep sleep mode, I’m aware of it but I don’t see how that would help in this situation? It is no different to having the Wemos power up and down based on the ignition state of the car (easier). It also means more complexity - if it were to be on all the time, then it would need a feed to the RST pin to tell the Wemos to wake up. “Having the ESP8266 in deep sleep mode means cutting with the activities that consume more power while operating (like Wi-Fi) but leave just enough activity to wake up the processor when something interesting happens” - but Wifi is exactly what we’re using here to identify home or away. With this, there is no PCB or electrical components involved, just Wemos with USB power cable.
Regarding License plate recognition, that would obviously require a camera (more cost), and also the vehicle would need to be parked in the same spot each time for the camera to take an image to assess the plate. If you parked your car in the garage every time, then you may as well just use a PIR. With the Wifi idea, the vehicle could be parked anywhere near your house (garage, driveway, road) and assuming good signal, would still work.
The idea of this was simplicity. IMO, as soon as you add too much complexity to a project, there is also too much that can potentially go wrong.
Hi All,
A significant amount of time ago, I created and sold a few of these.
https://www.nwe.net.au/ha/VA.html
Basically my Misterhouse home automation used X10 equipment as one of its interfaces.
When the car started, it continually transmitted a unique On code ie A3:On, after a few minutes, it transmitted a unique off code ie A3:Off.
The HA code rules then detected Neil’s car (A3) as leaving (A3 On code) and when it detected the Off code ie car was about 50 to 100m meters away, it picked up that Neil’s car was arriving.
So whilst driving around town, my car was always transmitting a A3:Off code but given HA at that period, I was probably one of three people within 100km that used X10 so it wasn’t much of a spamming issue.
I should make a new version.
Neil.