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.