It would appear that you have two objects grouped together--one of which is invisible.
My guess is that you inserted the tick mark graphic and then used button Setup to turn it into a button. This action groups an invisible "button object"with the visible graphic object. Both members of the group can have individual object names and Hide Object When expressions.
To see and modify the remaining Hide Object When expression, ungroup the button and you'll be able to get to the Hide Object When expression. Unfortunately, this also destroys the invisible button object and thus you have turn it back into a button when you are finished modifying the Hide expression.
Correct ! - thanks for the tip on this behavior
Hint:When dealing with this rather annoying problem where you have to destroy the button set up in order to change settings, it's often useful to make a copy of the button so that you can use it to double check the button options when you start to set the layout object back up as a button.