Usually it is on the logic board, but on this model the board is obviously in the middle of the Macbook, so it does not have the hall effect sensor here.

The only thing I can think of it being is a small circuit on the back of the keyboard, under a shield (see picture). I am not 100%, but having repaired a number of these devices, I cannot think of anything else it could be.

EDIT: Had to do a screen replacement on one today, so I took a clearer picture of the components under the shield. The circuit is very similar to the lid sensors on older models, so I think that is it. This is on the keyboard itself, so you would need to swap the keyboard if this is the part which is bad.