It is a "question" indeed.
Best I can see is that since a Chaos Sorcerer can use magics of different colours, that Grey Order Rank 1 spell for hiding spellcasting etc. is very handy to them. In 2nd edition, the Shades of Empire hedge magic also had a "hiding my magical ability" spell for hedge mages.
Though generally, this risk explains being loners and not hanging around "scene of the crime".
Wizards generally being rare outside major cities, an even in major cities in the "hinterlands" beyond Altdorf, Nuln, Middenheim, is another way of explaining it.
My house rules gives stress on any chaos star from magic sight to discourage spamming it.