It's called 'retail face' and you are afflicted with it! It happens to me all the time. I've worked retail all my life and every once in a while I'll be in a store somewhere (not where I work) and someone will approach me and ask 'Do you work here.' hate it!
I must have a real nice retail face. They usually just assume with utmost certainty that I work in whatever store I happen to be in and just walk right up and demand help. It's usually little old ladies. I think I'll stop being so nice and kind and getting things off high shelves for them and instead just yell "Scram, ya old bat!" at them.