IIRC, both, DD and D2 had a system that permanently stores the loot item on its place unless there are major changes in the location. Well, latter applies to D2 only. I don't remember if items disappear if you leave the location and come back, but they definitely disappear from Broken Valley after invasion.
As for DD - I think I still have old savegames from 2002-2003 with tons of loot dropped in the town and I stored them like that since beginning of the game. They never disappeared. Something like that would be a good compromise, no?
