Ok, if the system has been explored previously and all planets showed on entry to the system you may not have bothered to FSS it....
I am referring to specifically searching systems for desired preferred colonization candidates. Not just bouncing around system hopping, but actively doing FSS on a system (required to know the determine planet types) to find specific bodies with certain characteristics and the desired number of slots. I think the OP is referring to this too.
A certain amount of bare minimum effort is required to find a good candidate system, but is well worth it if planning to put +40 hours of colonization effort into building a set of facilities with a desired outcome in mind. Following the search, claiming the system, placing the beacon, building the primary port, then building a surface port it seems weird that an experienced cmdr then discovers the planet has bio or geo signals never noticed before. In my case a mistake? Perhaps, maybe. But unlikely.