Since you are working with Native and you are scanning an image then, you should see an orange rectangle. However, you also mentioned that you are using our sample Instant Tracking. Instant tracking is an algorithm that, contrary to those previously introduced in the Wikitude SDK, does not aim to recognize a predefined target and start the tracking procedure thereafter, but immediately start tracking in an arbitrary environment.
These are basics things that are covered in our documentation here. So I would suggest that you have a look at our documentation in order to understand how it works.