A search on tag terms sometimes returns no results when there are datasets with those tags. Example in wsc-reach with dem, lidar, and other tags (not the ones with a leading space, e.g. " dem". I suspect this is an error handling issue (I know that if a collection has that tag, the search results are empty because only data sets are expected and an error is thrown) - it could be that some deleted data sets have these tags, btu I have not checked the logs yet.