This line doesn't make sense.

Did he narrowly *miss* being discovered? Which is the way the word 'narrowly' would normally be used in this context.

If so, how would anyone know?

You can't narrowly miss making a discovery and also know about the fact that you've missed discovering. Only the person that narrowly missed being discovered can know about it... and even then not always. So, unless we have documentary evidence from him, which seems unlikely, we can't know if he narrowly missed being discovered.