I once worked with a fridge that would bleed. It was very inconvenient. We didn’t want it to bleed, we couldn’t figure out why it was bleeding, or even how it was bleeding, but there we were cleaning off this mystery red substance dripping from the door.
Rust. I’m guessing at some point it got rusty, and someone painted over it, and maybe there’s a bad seal or something so it’s condensation between the layers. Or maybe not even paint layers, just condensation inside the door.
Perhaps some alcohol that might age nicely.
Pictures of the house as it appears today.
a lengthy confession.
How about one of those plastic Halloween skeletons?
Some fellow who insulted you.
In my own bathroom I am considering leaving a copy of the Bathroom Reader Plunges into the Unverse Book VOl III. ITs already in our naked wall, tucked within beneath the exposed outlet.
Its the one with a story about aliens and ladies underpants. It really gave my 10 yo nightmares for a few days. GLowing panties sniffed by aliens it would freak anyone out. So I’m thinking of leaving that edition in the wall as we remuddle the place.
Photos of all the rooms in the house, or just a picture of that wall with the big hole in it.
Well yes, I’m sure it has some logical explanation, but seeing that at the time it was bleeding it was being used in a murder mystery play. I like the blood story better. 
When we redid the bathroom, and papered over the old medicine cabinet hole, I put a pair of Batman and Robin figures inside. Some fastfood joint was giving them away. Right now, you can get a full set of Shrek the Third figures.
You might try something you figure will be obsolete and long forgotten when found. For example, the short-lived serial comic strip Pibgorn recently fizzled out. See if you can find some ads for Drive, the adventure TV show that recently crashburned after only 3 episodes. You can get flashy brochures for new cars at any dealer, for free.
Bones. Lots of them.
Step two: put your dick in the wall.
CURSIN’ SWEAR BEAR
You just need to find a way to make it talk once a day or so…I imagine an electrically proficiant person should be able to find a way.