The only other passage where cedar wood, hyssop, and scarlet wool appear together is Leviticus 14. That chapter describes another ceremony (also "outside the camp") that resulted in ceremonial purification.
Some scholars suggest Eleazar added cedar wood and hyssop for their aroma. Hyssop is associated with purification (Exodus 12:22; Psalm 51:7; compare Hebrews 9:19). The scarlet yarn, like the red heifer, probably symbolizes blood. Together, the cedar wood, hyssop, and scarlet wool symbolize cleansing.