I did a search through all of my mail a few weeks ago. The search results "folder" is still around, despite my best efforts to get rid of it. If there is a way to dismiss it, that way is not at all obvious.
This is a "saved search" or "virtual folder", and the idea is that as new messages arrive the results of the search are dynamically updated. You can name the search folder in the search dialog, and so have more than one saved search. You're right, there's no obvious way to dismiss it. Well, maybe you're supposed to delete the search folder, but this is buggy: see bug 299937.
This bug has only been reported for versions before 4.14, which have been unsupported for at least two years now. Can anyone tell if this bug still present? If noone confirms this bug for a Framework-based version of kmail2 (version 5.0 or later, as part of KDE Applications 15.12 or later), it gets closed in about three months.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.0 aka 15.08), please open a new one unless it already exists. Thank you for all your input.