I can't imagine that people would forget that the people guarding the city walls could potentially turn on the citizens, so how could they be within the city and not be supposed to not be there due to the sacrosanctity of the pomerium?
I have to admit this seems to be a pretty cryptic question to me. Are you thinking of a particular event?
I might be able to help (tomorrow, unfortunately, as it's late for me now) if you can explain this question in a bit more detail.
Can you explain what you mean? Who are the "people guarding the city walls" that you're thinking about? Are you thinking of the Praetorians?