OMG When that police woman asked Dean who Claire was, Cas stood right next to her. As Claire is the offspring of Cas vessel they have similar features BUT Dean answered which very likely led to the fact that that random police woman thought Cas and Dean were not only partners but PARTNERS. In addition: Cas and Dean bought Claire a birthday present ( presumably together ) and I bet they must have looked like a couple to the other costumers and the clerk. If everybody else sees it they can’t not see it, can they?!