An unattended pack on a campsite could just as easily be interpreted as "they exited in a hurry and forgot this pack."  A set-up camp (although unoccupied) is "claimed."

Funny this came up while I was out on a trip.  One of our campsites was on the southern end of an island on Brent (Quetico).  We spent a few minutes talking to another group that was occupying the campsite on the northern end of the same island.  I kind of felt the urge to move on to claim the southern site, so I cut our conversation a little short.  We hadn't been on the southern site for more than 10 minutes when a tandem group (husband/wife) showed up looking for a campsite and a friend out doing a 51 day solo.  They wound up using a less desirable site just across from the northern campsite and then took the northern site the next morning when the other group departed.  

We also had a tandem group beat us to Ted Lake by a few minutes ... we took a longer route from Gardner Bay than they did and they were already on the "5 star" site when we arrived.  However, they were a couple of nice guys and permitted me access to their site the next day to read a few messages in the cache there and put in my own note.

dd