Yup, all enemies should have a close range proximity detection that will aggro regardless if you are in their field of vision or not. Additionally they should link to anything that they are friendly to that aggros and is in this close range proximity detection. It makes sense that enemies are going to react to you if you invade their personal space regardless if they can see you or not.

The problem you mention as well as the issue in CM and other places where you can run behind a guard and pretty much tap them on the shoulder without them reacting is dumb.