One thing that's got me out of trouble more times than I'd care to admit is a strong magnet on a stick.
I always put a mat down to catch anything that drops, but even so the ability of small fixings, screws & nuts to bounce and disappear is quite remarkable.
I thought I'd lost the retaining cup that goes with that pin and spring when I did my rear brakes last month. That little spring can fire it a surprising distance! Also the front caliper lower slider pin - it wouldn't budge at all, then shot out and vanished.
Magnet to the rescue on both occasions.