I can't be alone here. What unusual things have you come across hidden in wall, attics etc?
I once, tucked away in an attic, found a hollowed out African Elephant's Foot. The work was obviously done by a taxidermist. The homeowner had lived there 20 years, and never knew it was up there.
Another time, during the demo of a wall, I found an RG 22 (read cheap) revolver wrapped up in a shop rag. The frame was made of diecast zinc, and the cylinder to barrel gap was so big that it sprayed as much lead out of it as at the target.
In commercial / industrial locations, I often find a trail of empty whiskey bottles, or (in the pre-internet days) collections of pornography.
In residential locations, there have been the various animal traces. The animals themselves have invariably taken one curious whiff of the intruder (me) and scurried away; the exception being wasps. To be fair, our "possum" over here is somewhat different from the Kiwi version. (Ours is technically called an Opossum).
One house disposed of their old carpet by laying it in their crawl space- and using the area as a play room for their little kids.
I have one industrial site that is infested with copperheads - a poisonous snake, comparable to the old-world asp.
All I find seems to fit into three categories: 1) Trash left by previous tradesmen; 2) Amateur electrical work; and, 3) Evidence of illegal drug use.
Fried rattlesnake in a fire pump motor, live king snake in a well house, countless snake skins in attics, under buildings,and even one in a ceiling fan motor. We are always finding old tools and bits of iron every time we do any excavation. Of course wasps, hornets, and yellow jackets are real common here in Georgia. We have recovered at least 3 metric tons of trash left by other trades, old bottles, and once a Life Magazine dated 1956.
Life is tough, Life is tougher when you are stupid
What about creative insulation? When the floor was replaced in the upstairs apartment I thought I saw something sticking out underneath the wooden wainscotting by the window. Tugged on it and out came more or less clean (dusty of course) stockings and newspapers from the 1930s! I pulled one piece of newsprint to find a date and left everything else.