Is my secretary rude or does she just not know any better?

I have a private office at work. My secretary has a key for my convenience so that she can get things from my office when I am not there; however when I am in my office with the door locked she uses the key to enter without knocking. This is very annoying and rude as far as I am concerned. I believe she should knock first and wait to be invited in. If I am not there then she should use the key to gain entry. I would think she should know better. How can I tell her without sounding condescending