Last power outage, a garage door opened unexpectedly upon power restoration. I noticed since I was home. The hub was on a TalentCell backup battery. I attributed it to the ST Arrival Sensor losing contact with the hub with the loss of power and associated repeater devices, making the hub believe a car had left. Then, when the power came back, the hub thought the car had arrived and since it had the juice to do so, opened the door.
Anyway, my wife said she actually liked the door opening when she pulled up. I don't get a whole lot of consistent praise about my home automation work. So, I thought I'd try to give it another go.
I figured, I'll make the garage door close 15 minutes after arrival. This way, we can open the garage door manually and keep it open while we do stuff and the doors won't close. The timer will only be tied to arrival. I have it close all garage doors when a car leaves, but that's only an occasional minor annoyance.
So, I'm back to the 4-6 week battery replacement routine on two cars. The battery hack would have the arrival sensor down low in the car (signal for long driveway) and I'd have to figure out where to put it. I guess signal wouldn't be an issue with @iharyadi 's solution, but hooking it all up so it looks good might be an issue. I'm going to maybe look some more into smart phone presence sensing, but it seems like nothing is totally satisfactory there, especially since I keep mobile data and location shut off most of the time.