@Reynard and
@Umbran.
If people are using investigation as a skill for deducing information from a given scenario then, yup, I agree. Treat it separately to perception. (Minor nitpick, but maybe rename investigation to deduction or such.)
But some people* do not use investigation that way, or exclusively that way. Some make the distinction of perception being passive and investigation being active, as
@Chaosmancer implies in their post #439 above. A distinction that, IIRC, was made in 3rd ed. where the rules were first introduced to DnD.
I personally do not consider such a distinction all that distinct.
* A lot of people in my personal experience, for whatever that's worth.