A thief (or a pen thief).
This is the type of riddle where one is made
to think the riddle is much more difficult, so much so that a person
overthinks the riddle and either gives up or provides an answer that is
too outlandish and overthought. The correct answer is so simple, people
neglect to consider the obvious that one who has no pens at arrival yet
is able to leave with six pens just might have stolen them.
