For Context: So I am performing some user testing, and the interface is changing how tabs work with in the system. Currently tabs indicate the top level of different customer define hierarchies. The new interface is changing tabs to act like browser tabs, as in, the tab indicates the location you are at allowing you to leave tabs open and jump to different levels in the hierarchy or hierarchies.
Note: image above is cropped version of the wireframe
I expected the users to have some trouble initially as the entire mental model of tabs is switching from what they are today (top level locations only) to any location. What I didn't expect is that users seem confused what the + sign next to the tabs indicates. They have (3-5 users) indicated that they thought that it would show them additional tabs that are already open. In the tests, however, there has only been one tab open at the beginning of the test, which would seem counter-intuitive as there is a large amount of white space to the left of the tab.
The Question Is this a case of, in this case rely on the "strong designer" design model and stick with the plus (as the mental model is supposed to emulate how the user already uses their browser)
Or is my test really revealing that i need to replace the icon with something else. If so, whats your opinion on what icon screams "Add New Tab"