The red fume can be seen in some ballistic missile tests and of course here for the case of Chinese KZ-1A orbital launcher. i wonder what's the usage and why it leaks.
Red fuming nitric acid is used as a storable non cryogenic oxidizer. It does not leak, its fumes are vented, I guess to prevent a tank overpressure.
From this pdf Problems in Storage and Handling of Red Fuming Nitric Acid
So there might be too few NO2 and H2O within the mixture. Adding water would be dead weight, but 2 to 3.5 % should not be a problem.