Scenario 1:When we encounter with a deviation with the behavior w.r.t functionality or GUI as specified in the FRS or GUI specs then we can directly report it as Bug in the Bug Tracking Tool.It is always a good practice to take the screenshot as a proof of evidence to show to the developers that the bug was there in the application when you were testing it if in case the bug is not repoducable at his end. Specify the steps to reproduce the bug if the bug is reproducable.

Scenario 2:When we are not sure whether it is a bug or not we can send it as an observation to the Test Lead he will review it and ask us to report it, when he finds that the observation can be reported as a bug in the Bug tracking tool after refering to the documents or communicating with the customer regarding the behavior of the bug and getting it clarified whether it is behaving as intended or is there any deviation in it if there is no proper documentation .