If you are obsessed like me, you will want to discover all of them by yourself and that means you should not read this, as it is a potential spoiler for you guys/gals.
However, there are more than 90 secret locations in Fallout 4 and you may not want to go so hard on yourself. Youtuber The Lawd, shared more than 90 videos, all showing the secret areas of Wasteland. By secret area, I mean the locations which are not marked on Pip-Boy's map.
You can check out one of the videos below.