Here is the section of the M-41 which tells us to keep the mail secure. It tells us when we see mail with uncancelled stamps we must take the time to line through them. We are not allowed to put mail in other mail at the case or on the street. (Many carriers do this because it's a short cut...faster. There isn't anything in our M-41 that ever tells us to be faster over precise and thorough. This point can't be overemphasized enough. Remember, you can't get in trouble for following the rules.) Also, when we find things in the mail boxes out on the street we are supposed to do something about it.

13 Delivery and Collection Rules
131 Delivery on Letter Routes

131.1 Security of Mails

131.11 Keep the mail in your possession or locked in a relay box or vehicle. All equipment and undeliverable mail must be returned to the post office at the end of each trip.

131.12 If the use of a satchel cart is authorized, exercise reasonable care to prevent any attempted theft from the cart while delivering your route.

131.13 When leaving the route for lunch, lock undelivered registered and COD mail in a designated relay box or vehicle in accordance with existing practices. Make a simple record of registered, COD, and other more-than-ordinary value items which are left. On return from lunch, check to insure that no pieces are missing.

131.14 Do not remove stamps from mail or throw away or improperly dispose of mail. Line through uncanceled stamps (except on philatelic mail) to prevent re-use. Delivery Point Sequence mail may contain uncanceled stamps that will not be identified until the mail is being readied while performing street duties. These stamps should also be lined through (except on philatelic mail) to prevent
re-use.

131.2 General Precautions

131.21 Do not deliver mail that has not passed through a proper delivery unit.

131.22 Do not place letters, large flats, and other mail within the pages of magazines, catalogs, etc., at any time. Whenever letters and flats are cased together, care must be exercised to avoid having letters fall between pages of magazines, etc.

131.23 When you find in a mail receptacle mailable matter on which postage has not been paid, addressed to, or intended for the person in whose receptacle it is deposited, take the matter to the post office for treatment as provided in section 146.2, Postal Service Manual. If general distribution of the same bill, circular, or other matter has been made on the route, return only 2 with a statement of the number of similar pieces without postage.