For example, we have you pass a wisdom (history?) check to know about The descent into Avernus when u talk to Zevlor. That was a huge event, but apparently if you are not a history buff, you are oblivious. If we are imagining that everyone we encounter is constantly making these rolls, then it is totally plausible that some portion of them would be oblivious to the Shar symbol, as it is not something they gave encountered or can recall if they had.
