Reading output of LED driver with Arduino

I have a few ESP8266 hanging around the house that control lights via websockets. We're deep into the Google Home ecosystem which is a pain to work with. I would like to control my ESP's via google instead of my own app. My brilliant idea was to use one of our left over google compatible LED strip controllers. Set it up and it works just fine on its own. Now, I wanted to read out the level by simply hooking it up to a voltage divider and a capacitor to smooth the PWM and reading it via the analog port, and then broadcasting it to the ESP8266's. My problem here is reading the output. When I turn the LED controller on it reads 2.9v (as expected) at the input pin. But when I turn it off, it reads 12v between everything for some reason...each leg of R1 and R2. I am at a loss here, I got NO IDEA why it behaves like that.
Also, when I hook up an LED strip to the controller and hook the negative into ground the same behavior happens.

How can I read the output of the LED controller? I'm also not opposed to using a completely different approach!

Thanks!

(very crude) schematic for illustration purposes

level of what? PWM? it is 12V.
How is the LED controller arranged? is there a diagram?
where is LED Output connected? it is clear that R2 will from GND disconnected.

What controls the LED controller? Would it be possible to get data from the controller controlling the LED control?

A good starting point would be to understand the relationship between the LED ground and the ESP ground.

In all likelihood, they should be tied together.

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