Sorcerers pretending with the book don't make NPCs "freak out" however, each casting does have a chance (based on the number of observers and if any of them have wizard levels) of raising the character's reputation a bit.
I think this is a rather creative idea. Maybe sorcerers could make a preform check proportional to the level spell they cast. Something like DC 20 + (spell level/circle)? If they pass they go unnoticed, if they fail NPC's would react much like they do now. A check is made for each NPC in range. They generally have a high charisma so i could see this be a plausible mechanic.
I think to really fit the feel of what the DMs are talking about, casting sorcery needs to always be a risk. To fit that, it can't be something the character can just work into always succeeding at.
Having the NPCs make the check vs something like DC 20-(spell level) with wizards getting a bonus to the roll (maybe + highest spell level they can cast) would fit that better.
Maybe if they make the check, they eye the character suspiciously and one of them makes a inf check with a bonus for each npc that made the first check. Something like that.
As long as they're never sure to get caught, and never sure they'll get away with it, you create an interesting dynamic where the book is always helpful, but never a guarantee.
As for the sorcerer, they already have both their CHA and their Inf dampening damage to their reputation, so there's not much need to have more of their stats helping.