Searching "security" on our blog comes up with a number of hits for things do with and ways to admire the beauty of the patterns inside security printed envelopes. A security envelope is an envelope that has a printed pattern inside to keep the documents within away from prying eyes. Through a tweet from stationery purveyor and Instagram genius Present&Correct, I found this site which is attempting to catalog all of the patterns found inside security (also called "safety") envelopes. There's a nice history of the envelope in the About section as well. Do you have a favorite? Is it in the catalog?
P.S. They also linked to this listing of artist books featuring the patterns.