I once found a bunch of razor blades in a wall I was busting up, wondered what the heck that was about until someone explained the hole-in-back-of-the-cabinet thing Sven mentioned.
One house I didn't find anything interesting in the walls, some unpleasent surprises in the cielings though! Taking a break from electrical work I was helping a friend and contractor tear 3/4" ply off the cieling, it was coated with a nice layer of rat, err excriment. In that same cieling I found a pair of unprotected #2 wires (not a surprise with the way this garage was wired when it was turned into a living space- unprotected THHN used for everything) what WAS a surprise was that the wires were hot, after the meter was pulled! We installed a new service, and left the old one, that ran inside a soffit for about 50' so I never did get to see exactly how they did it.
Same house, fishtape gets hooked exploring some old empty conduits, so I went into the attic, and saw several of those glue-type mouse traps, with some nicely mummified mice still stuck in them, thankfully they were past the decay phase.
