Is there a law in Arkansas against searching employees cars as they leave the company premises to check for stolen items?

The legality of the matter may be secondary to whether you wish to remain employed or not. Arkansas is a 'right-to-work' state - which means that (except for certain industries) you can be hired or fired at the whim of your employer. If part of your employment is being subject to unannounced searches of your lockers, and automobiles (while on company property) I would tread carefully if I were you.