A user with the Delete'Ticket' right can change the status from any ticket in a queue to the deleted status (Display a Ticket, choose Respond/Resolve/Comment, select deleted status in combo, then Update).
This right can be granted to users or groups on a global level, or on a queue-by-queue basis.
Deleted tickets are almost impossible to find if you do not know the number. On most RT installations tickets should only be deleted if they should never have been created in the first place; e.g. a spam message coming in via E-mail.