Urine is also a good wood stain. Many old flutes and recorders were stained by leaving them immersed in urine for a few weeks or months. If you have an old boxwood instrument that isn't yellow, that's what's been done to it.

I've seen this effect in houses in Edinburgh: one I lived in was built in 1826 and the bed had been in the same place ever since. Where the chamberpot would normally go there was a dark circle on the floor. You find this fairly often where the original floorboards are still in place. It also happens underneath the baby's cot.