Desdemona
I have heard it a few times and find it patronising, yes.
What I despise too is being called "my darling" or "my love" by shop assistants etc. Do they only do this to people of mature age? Winds me up A LOT!
I'm not sure if I have ever heard it said or just seen it written, but either way, I can't imagine ever saying it myself (though you never know), but I don't think it is necessarily patronising - depends how it's used, really.
Regarding things like 'my darling', I remember frequently being called things like that by (mostly male) shopkeepers when I was a young woman, but as they did exactly the same to all the other female customers, whatever their age, I didn't have a problem with it.
More recently, I was shopping with my elderly mother when the male shopkeeper made what (I very much hope!) was a joke about us being sisters. I couldn't help telling him that his comment might make one of us happy but definitely not the other.