Apparently, when I finish with a tool or part, I just throw it over my shoulder. So it can be in literally any part of the shop, even if I never stepped away. There are two good things about this:
1) no need for frustration, just start a whole shop search. None of that “where did I use it last” nonsense. Just wastes time.
2) In the search I will likely come across some shiny thing that triggers a brilliant thought requiring immediate attention. This completes the search entirely. In the course of the new project, at some point a new whole shop search will occur, and the original missing part will be discovered, generally right where it was left. This also terminates the previous search. See?
I feel like this is revolutionary, and more easily implemented that all your above suggestions. I should write a paper on this, maybe I will when I come across that darn pen.