Great post by Kevin, made me think:
It's not radiator keys with me, I sorted that by buying a dozen from Toolstation and secreting one in every toolbox or drawer I could think of. Doesn't completely solve the problem, but greatly increases the chances of finding at least one.
With me it's pencils. The very second I put one down it vanishes completely. I buy boxes of 50 from Staples, but I can still never find one.
Some manufacturers had the bright idea of making tape rules, stanley knives, levels etc in bright fluorescent colours so you could find them easily. I now have a toolbox full of brightly coloured tools and still can't find what I want.
I also solved the 'which power tool is plugged in where' mystery by attaching coloured insulating tape to each end of the flex. So the circular saw has red tape each end, the sander yellow/blue etc.
In another desperate attempt to get organised I keep each power tool in a suitably marked storage box. So the jigsaw box has all the bits; fences, blades, spanners, hex keys etc to go with the jigsaw. Manufacturers carry cases are often lacking in this respect. Plenty of space for the power tool, no room for the discs, blades, abrasive etc.
This Handyman malarkey has taught me a lot. Every job is different, needing different tools, nothing like doing a job at home. I solved this (hopefully) by having a 'standard' tool box with the stuff I know I'll probably need every day: level, tape, drill driver, adjustable spanners, screwdrivers, square, stanley knife, mole grips, saws, hammer, pry bar, clamps etc. This is supplemented by an organiser containing screws, fixings, drills, bits etc.
Then I have boxes for various tasks like plumbing, decorating, tiling, laminate flooring, electrics and such like.
I still occasionally arrive on site without the required kit, not too bad because I work in a fairly small but densely populated catchments area, so I can always go to a B&Q or back home if I need to.