There are some good options here, but until you know where it is I don't think you will know which one to use. Compressed air or a leaf blower might drive the thing into a place where it is inaccessible. And you can't use one of those flexible grabbers unless you already know where it is. Some of those grabbers have a tiny LED in the business end - that might help you find it. Failing that, a pen light that is small enough to maneuver into tight spaces. I really think you need to find it before you will know what to do.