A shredding service issues a certificate saying they destroyed the material given to them.
Does a Certificate of Destruction guarantee my documents were destroyed? What if something I thought was shredded turns up later?
There is no guarantee with any type of service. Recycling companies have been known to issue Certificates of Destruction because the material does end up being destroyed in the recycling process. Since any shredding service does not know what is given to them or if there are other copies, they can offer no guarantee that any particular document was destroyed. The only way to be absolutely sure is to shred yourself or to use an On-Site service where you can watch the material being destroyed.