Decent vodka, being basically ethanol, is tasteless and odourless, making it the drink of choice for alcoholics etc. So how did the passenger "smell it on her breath"?
Considering that the means of detecting intoxication is by measuring the %age of alcohol per volume of exhaled breath, it is easy to smell the alcohol on the breath of someone with such a high concentration - irrespective of any mixer or other flavourings / aromas the drink contained.
Anyone who can't smell such a high concentration of alcohol on someone else's breath either has a medical problem, or, is more than likely intoxicated themselves.