Sometimes I can hear of see where the part landed, but then it takes another random bounce or roll. Often I'll buy $5 worth of tiny parts for some weapon while I'm online buying major stuff at Brownell's. Some of these tiny parts cost under a dollar, if you have them on hand, so adding them to the list might save my butt at some future time.
Another good policy is to buy the "rebuild kits" that contain every single small part, pin, and spring to make something work again. This is a good idea, especially if you have more than one of a model. But buy a few more parts than that of parts you know you lose easily.
This goes double for classics that aren't in production any more. If it does go down, you'll take it easier if it was a major part that you could not predict, rather than nickle-dime parts you could have bought for next to nothing ages ago.