When are Archived Redo Logs generated?
Oracle itself will generally generate redo in the process of running various background jobs.  For example, by default in 10g, there is a job that gathers statistics on objects.  That will generate redo.  By default, 10g has a job that gathers AWR information as well every hour.  That will generate redo as well.  There are a number of other background jobs (checkpointing, updating the SCN to timestamp mapping table, etc.) that are going to be running as well which will be generating redo on an ongoing basis just as a matter of course.

If you run the query 

select current_scn, systimestamp 
  from v$database


in a (from a user standpoint) idle database, you'll note that the current_scn is incrementing every few seconds.  That is Oracle doing various sorts of housekeeping.  That housekeeping will generate a small amount of redo each time.  Over the course of a full day, it's pretty common that this would add up to enough for a couple of archived logs even if your system is otherwise pretty idle.