RPM, were it me in your situation, I'd continue to do my job to the best of my ability while looking for another job. It's apparent to me that someone there, either the store manager or the store detective, doesn't like you, for whatever reason. It'd be best to get another job elsewhere before they fire you for some trumped-up reason. That kind of thing can follow you and make your life much harder than it needs to be.
This, according to One-Who-Shall-Remain-Nameless, is my "valuable" opinion.