Never lost in the post, but I once lost a film that I knew I'd left in our kitchen. It was Autumn when I lost the film, and spring when I found it under a tree in the garden alongside a number of teaspoons.
I worked out that our cat's magnetic devise on her collar that activated the cat flap, was carrying small metallic items out to the garden that would then be deposited below the tree as she jumped up into it.
I had it developed but alas nothing came out.