1) Make sure that there isn't anyone around who owns the pack.

2) If conditions permit, conduct a search to see if there's an injured person or even a forgetful hiker taking a nap nearby. How wide the search radius should be would depend on the terrain, conditions, and how many are in your party.

3) Carefully note the spot where the pack was found, using a map, GPS or other means to ensure that you can find the exact spot when you return. Blaze a tree, make a stack of three rocks, or something of the sort so that you can be sure it's the right spot.

4) If your cellphone works, and there's enough information to do so, call the person who owns the pack. If that isn't fruitful, call the appropriate authorities to tell them what you've found and where.

5) If your cellphone does not work, bring the pack to the authorities or to somewhere that your cellphone works. Leave a note stating that the pack was found and where you're bringing it.