[helb]

I think the point was that the large moving metal object (garage door) itself would mess with the signal enough to detect the state without any additional device…

[detaro]

I guess a first good experimental step would be to point a directional Wifi antenna in the direction of the door and monitor what it receives at what strength.

[TeMPOraL]

Yup. And the actual implementation difficulty is in setting up the detection/analysis stack.