Panel, in NGUI's terms, is just a script that draws widgets. Panel has no "rect".
Now if you create an invisible widget (ALT+SHIFT+W), then it will have a rect and you can place a collider on it (ALT+SHIFT+C), thus allowing it to receive touch events.
If you want to check if your touch is within the UI, easiest thing to do is to check UICamera.isOverUI -- this will work with NGUI's events, such as OnDrag, OnHover, OnClick, etc.