If it doesn't smell like bacteria it's probably not bacteria. I studied and worked with bacteria for over 5 years when I worked in the medical industry. I can't think of a single bacteria that didn't have some sort of organic stench to it. Some smelled like grapes (seriously), some like putrifaction, but they all smell like something you wouldn't want to put in your mouth. Except bacteria used for cheese production. Some of them smell ok …

Anyway, you probably know what your coolant used to smell like. If that smell starts to change in any appreciable way and shifts from smelling non-organic/oily to organic/spoiled than I'd be suspicious.