There is nothing like hidden topics in IAS exam but yes the questions sometimes not come directly but indirectly. The exam paper is mold in such a way that it looks like a tricky. You no need to do anything special just go through the syllabus well and NCERT books. Past some 1 year question paper. If you will join some coaching institute then you will automatically get complete knowledge about the particular subject. You will get sample question papers by which you will get complete overview of questions to come.
Joining coaching institutes result in better preparation of exam like Chanakya IAS Academy and Sriram IAS.