Could anyone tell me what setting to change?
I keep seeing a thick outline even if my shapes have a thin / no stroke.
Below is an image showing a line with no stroke or fill yet the line is still black
Did you check the Appearance panel for additional strokes?
Can you show the entire Appearance panel?
… and also check the parent Group/Sublayer/Layers Appearance.
Please show a screenshot with selected item and visible Layers Panel.
In addition to checking Appearance for the object, check Appearance at the Layer level.
Check the Appearance of your layer "Outlines" in the Appearance Panel. (Do you see the filled circle rght beside the layers name in Layers Panel in your last screenshot?)
how do you check the appearance of a layer?
Click on the filled circle right beside the layers name in Layers Panel
why did the stroke apply to the whole layer?
>>why did the stroke apply to the whole layer?
That's what that feature does. It applies appearance settings, including effects, to any on that layer. Move something to that layer and it will suddenly display the settings.
… mostly by mistake