Is it legal for a cop to walk through the back door of an establishment, during business hours?

At work this evening (work: locally owned restaurant) a cop just came walking in through the back door, we all said hello to him to be nice and the only explanation we could get out of him was because he was bord. mind you our back door leads stright into our kitchen-also it was during business hours. If you or someone you know is a cop please tell me if he was in the right or wrong.