I was wondering if there is a site or repository of standard UX questions used in the Discovery phase of Website/Portal/Collaboration site.
I can only assume that you are referring to heuristic evaluation questions. Here is a great guide to conducting an evaluation: http://www.uxforthemasses.com/usability-reviews/
Here's a great list of Heuristic Evaluation Questions from the Society for Technical Communication's Usability and User Experience special interest group. They have some questions that help you evaluate discoverability as well as general usability concerns. None are explicit "is this item discoverable?" questions, but they cover the low-level concerns that make up a discoverable interface such as
Is only (and all) information essential to decision making displayed on the screen?
For more high-level thought about the issue of discoverability consider Scott Berkun's The Myth of Discoverability article, which covers why you can't make everything equally discoverable and how to decide what should be most discoverable.