I've never actually done it, I mean leave my pack to indicate that I intend to use a site.  But I would certainly honor such an act.  Typically when I'm paddling towards a site, if I see color - bright blue, red or green -  I just assume there is someone there and move on.  It would never occur to me that someone simply forgot their loaded pack and paddled away anymore than I would assume that they had forgotten to take their tarp or tent down when they left.