actually Bourbon had so many chances of getting outed as a NOC, even if we're not counting the movies
like, both of the people he most worked with (Scotch and Rye) were found out as NOCs, which would personally make me suspicious of him, and the members of the black organization are way more wary about NOCs
not only that, but Akemi Miyano is his childhood friend? she could have let slip something about him and his cover would be blown
there's the time where Taka'aki knew where he was and that Kuroda had to act too, because if he was too late to act, his cover would have been blown in front of Rum of all people
not to mention all the time he can be seen with Kazami? or the fact he uses the same car as Amuro Tooru and Furuya Rei?