If I remember correctly, long ago (around Luclin time?) mobs didn't regen so fast... since I was in one of those "wipes so often" guilds, and our nemesis were always standing by to engage our mobs after we dropped.
I remember vividly the NPC camping our corpses for at least a few minutes before wandering back to it's spawn point, and slowly regenerating it's health.
But lately, you are correct. Combat ends, mob sits there for 30 or so seconds, then runs back to it's spawn point fully healed. At least I think so. I haven't died in EQ1 recently.