It goes off all the time. And each time, my hands are on the steering wheel.
It doesn't actually detect contact - it checks to see if you're actively adjusting the steering wheel.
Except I don't need to! The lane keep assist is so good that it's rare I have to give it additional help.
So - I kid you not - I've gotten used to giving a nudge to the steering wheel every so many seconds to prevent that warning (you cannot disable it).
Imagine a car gave you cruise control, and then checked if you were paying attention by requiring you to press down on the accelerator every so many seconds. Does that make sense?