"things that you might not have noticed or forgotten"- does the 'not have' modify the 'forgotten'?

The sentence is ambiguous written that way.

It seems that it means to say, " There are things that you might not have noticed, or have forgotten."

You could substitute 'might have overlooked' for 'might not have noticed', which would mean you don't have a negative 'not' action followed by a positive action, and you could then use 'or' to combine the action, thus removing the ambiguity: 'things that you might have overlooked or forgotten'. Most negative forms have a positive form counterpart - a few examples:

did not do (something) - omitted to do (something), missed out doing (something) etc
did not remember (something) - forgot something
did not have (something) - lacked (something)
did not know something - was ignorant of (something)
could not do (something) - was incapable of doing (something)