It's a reminder for the postal carrier that delivers the package to scan it.
Here's how DELCON works.
Normally, they are scanned at the receiving office when the packages are sorted.
The delivering postal carrier has a small hand held device that is used to scan the package when it is either delivered, attempted delivery, refused, etc.
When the carrier returns to the post office, they place this scanner in a cradle and the info is downloaded off of it.
This info is then compared with the information that post office has for each carrier. They are given a score (0-100%) based on how many packages were scanned correctly.
Some carriers grew up in the 60's and have a little problem with the brain cells not holding hands all the time.
Therefore, they use these "SCAN" or "SCAN ME" stickers (normally bright orange) to remind the delivering carrier to scan the package and enter the appropriate info in to the scanning device.