People have been describing two different situations: holding a site briefly and reserving a site that you do not even stay in one or more nights.  The later is never acceptable, no matter what you leave at the site.

Leaving just a single thing, even a pack, is iffy because someone could think it was forgotten.  But leaving behind a bunch of stuff (or a pack where gear has been removed to some extent) clearly shows someone is occupying the site.  Setting up a tent is not required.  I can even see a scenario where someone going out for a day trip but returning to the same campsite might take down their tent because of concerns of what weather might do if the tent is left up (maybe the tent is in a location where it is hard to tie it down and the site is quite exposed to wind).