I guess all will agree that a PM Search is Needed for any forum.
It doesnt make sense to not have a way to search on the private messages.
I saw this extension: https://github.com/satanasov/pmsearch
But this really should exist on the phpbb core, imo.
Why? Finding relevant information. Since we many times dont know who send it but we remember some of the text (keywords) and with that we find the PMs easely. No need to check one by one.
We've an ticket and all lol: https://tracker.phpbb.com/browse/PHPBB3-14582