If you loaded the 4.3 OTA, you have KNOX loaded courtesy of Samsung. Read
here. I have not updated yet, I'm still on 4.0.4, but this 4.3 update has given me pause. There has to be some type of admin app that will allow installation of non-signed apps.
EDIT: Initially, I thought the article referenced did a decent job describing KNOX, however, the article ends with some references to XDA and a method to remove KNOX. I wouldn't use it. The article states you may lose data. I'd stay away from the removal method, but it gives one the idea of what KNOX is all about. Search AC articles, I'm sure they wrote something up a while back.