This used to happen to me a lot, partially because I would wear or carry a winter coat in the wrong season or bring a backpack everywhere, and it gives the impression you are looking to have a place to hide items. However, I may also have learned to shop with more normal body language because I started spending more time with people who actually like shopping... or maybe it's just that I started shopping less myself.
I was poor at detecting negative intent at the time, so I didn't know what it was and genuinely thought they were attempting unnecessarily to be helpful, until I had friends who worked in retail who talked about these techniques.
There is a section in a book I was reading about this, (Ask and Tell by Stephen Shore) where the author of the first chapter describes being selected for random frisking and additional scanning every time she went to the airport. Until one day, she caught sight of herself in the mirror and noticed she looked tense and nervous. So the next time, she copied the way the other passengers were standing... and made it through without fuss.
I think whether it's nervousness or that you're giving off different signals in some other way, it is possible to change the signals if you want. However, if you don't feel like it's worth it, I could get that too. It's annoying that so many neutral behaviors can cause so many problems because of others' preconceptions.