The Asset View can only show assets based on the following criteria: selected sources, then any filters and searches. For this reason, it is important to know that you must have the appropriate source folder(s) selected in order to see the assets within them.
In order to search for assets in multiple folders, you must select all the folders you want to search. The Asset View will not display assets of any unselected folders. This means that in order to search all assets within your project, you must have all project folders (or a parent folder that contains all of them) selected in the Sources View.
Like with folder and file listings in many programs and operating systems, the following key combinations apply:
Mouse Click - Replaces current selection with whatever folder was clicked.
Shift+click - Toggles a folder's selection status. Will also range-select all folders between a start and end point.
Ctrl+click - Toggles a folder's selection status, but does nothing to any folders that were not clicked.