I regularly do this and I think it's quite funny that some people have so little going in their lives that they might get annoyed by it. I will make an effort to do it more blatantly from now on.
My life is so full that I don't get much time to eat so I am often hungry or thirsty when I get to the shops. A nice big swig on a carton of apple juice is a favourite. All goes to the checkout in the end.

