1. Blow whistle to try an attract attention of pack owner.
2. Mark the spot on your map or GPS.
3. Take a picture of it.
4. Take a picture of or write down the ID information.
5. Contact the local rangers as soon as possible to let them know of the pack's location.

It's fairly common for people to set their packs down while they do a short side excursion, either of a steep slope to check out the view or to answer a call of nature. Taking a "found" pack is major bad juju on the trail. Try to find the owner if you want, but I wouldn't spend more than maybe 15-20 minutes looking. I am getting kind of crotchety in middle age...

-Blast