That is what I am whating for.
You seem to have read the WN or LN, the MC keeps secrets because he knows that having the hero job is considered treason by the empire and they will hunt him down and murder him for it.
Still, he probably SHOULD trust Roxanne at least and perhaps Sherry with a bit more than he does (maybe a lot more) but by keeping these things secret and vague he prevents them from accidentally exposing him while he deals directly with the very same people/organizations that would be directly responsible for seeing that he is hunted down if the true reason for his abilities are known.
For example (if the MC told them about his abilities), If the King or someone close to the king actually knows how appraise works and that is the direct result of being a "hero" or from "another world" and Sherry casually/inadvertently mentions to the elf slave or some other person that the MC is able to tell when smithing will work and/or can "see" skill slots.
Then the Elf or some other high ranking person (the MC doesn't interact much with low ranking people) may innocently bring this up in conversation and a rumor spreads until it goes high enough for the MC to receive a nice invite from the king for a "chat".