Whenever the probation officer has cause to believe that there was or is within the county, or residing therein, a person within the provisions of Section 601 or 602, the probation officer shall immediately make an investigation he or she deems necessary to determine whether proceedings in the juvenile court should be commenced, including whether reasonable efforts, as described in paragraph (5) of subdivision (d) of Section 727.4, have been made to prevent or eliminate the need for removal of the minor from his or her home. However, this section does not require an investigation by the probation officer with respect to a minor delivered or referred to an agency pursuant to subdivision (b) of Section 626.