I don't think there's a smarmy word for 'very vague warning sign' that's widely used. My friend group uses 'yellow flag' to mean basically what this woman means, which is something that is associated with shitty people, but isn't really a red flag in and of itself. Kind of just a 'hey, watch out for signs of the type of shitty behavior that can come with being a car guy' or whatever.

Or like, if someone's a GAMER, it might be fine, but if you hear them saying KEK and FREN, probably time to cut and run.