I was glad that I had seen and rectified my mistake "before anyone noticed". Or "before nobody noticed"
Some other sentences that would be correct:Here meaning is that nobody noticed it
I think this may be the issue. Nobody in fact ever noticed the mistake: so why do we say "someone noticed". How is it possible to locate an action in time before a non-existent event? I suspect that the point is that "before" has a special unusual meaning here, synonymous with "lest".Before "nobody noticed" is itself negative meaning the the action didn't take place. How could you have done something before an action that didn't tale place? You can do something only before something that happens.