Beating this espionage horse, I read the Bhuric description of 'Spy and Espionage Calculations' (or sumting like dat via da link) and I understand your explanation (I tink), so I send in four spys to a city with whose Empire I have closed borders (CB) and the first gets caught and the next three have successful missions (all four done consecutively on the same turn). Then I send in four spys to a city with whose Empire I have OB and get the same result. So, more spys is directly related to more successful missions (Bhuric's conclusion).
Then I got to wondering, since all the spys that are caught by my spys/Sec Bureau and identified are from the Empire with which I have CB, and since CB enters into Bhuric's calculations, and since the Empire with OB is the highest on the Espionage chart so it must be conducting espionage missions, could I correctly conclude that the unidentified spys that were caught were from the other Empire with OB? This may be s-s-strech-ing the parameters to fit my observations, but there's a heck of a lot more spys caught and no country mentioned which causes me to think that I have a bigger problem with my 'friendly' neighbor than with my 'cold shouldered' neighbor.
Now, if there is some way we could waterboard that unidendified alien sabeteur........
Thank you in advance for your response.