I used the term plot as the NPCs plotting their own agendas and not really relating to the PCs. My gnoll warchief for example is "plotting" to destroy the village by allying the other tribes. So for me an NPC plot is a note that explains the agenda of a PC. Most of my NPCs inside the sandbox have them. They might be very simple. I guess I could have called them NPC goals or agendas. Maybe agenda is good but that term can at times be loaded too.Great post, I entirely agree. I wanted to emphasize that it's the clues and hooks that get the players to the locations, not the locations themselves. I mean you can run a simple hex-to-hex sandbox but I find the breadcrumb style way more interesting.
One nitpick is the word "plot". I try to avoid using it. A DM shouldn't create plot. The plot is created when the players interact with the events and happenings that you've set in motion. It's semantics I know but it helps me get in the headspace of what my job as a DM is, especially in a sandbox.
It really helps me present things as a neutral adjudicator and then get out of the players' way. It's my job to present the world as it is and let the players decide how to interact with it. It's not my job to present only the story I want to tell.