Bethesda has always been good at sneaking in some hidden secrets into their games due to how massive their open-worlds are, and Starfield most definitely upholds this tradition, with there being plenty of them to seek out during a playthrough. The truth is, because a lot of the secret areas and items are tucked away, or simply just really hard to find, it's unlikely that players will come across them when following the main missions, or even the faction missions.