Found it!

A while ago a booklet that I'd put on top of a 2m tall bookshelf ended up falling right down the back.  Today I managed to use an inspection camera plugged into my phone to find out where it was, and then direct suitable implements to extract it.  Brilliant.  It's amazing what you can get for a few quid on Amazon.

Now I've worked out how to use it, there are quite a few other jobs involving nooks and crannies it should be able to help with...