10/03/2025
I appreciate everyone chiming in to offer insight into this situation, even those who offered no insight but comic relief - that too is appreciated. After reading through all the responses, I find the following to be the most probable solution to this mystery:
Hygroscopic. A word I'd never heard until this thread, so I did some research.
It turns out that urine is hygroscopic. So it's possible that the toilet overflowed & deposited its contents across the floor in a relatively uniform pattern multiple times, gradually seeping into the floorboards. This process could have repeated itself many times & over the course of years, I suppose, permanently embedding the urine & uric salts into the fibers of the wood.
From what I read, "...urine can chemically react with the tannins in wood, causing permanent dark stains. It seeps in, crystallizes, chemically alters the wood, and can leave behind lasting stains and smells." The stains & smells of urine damage are pretty much common knowledge. The key phrase there is "...chemically alters the wood...". The chemical alteration of the wood itself is irreversible, & the hygroscopic nature of urine & uric salts will continue to attract moisture indefinitely.
This would explain why the floorboard that I removed continues to become saturated even though it's no longer in the floor or in the same room as the floor or, at times, in the same house as the floor.
As for the ghost theories: I'm not yet completely convinced that something otherworldly is not at the root of this. Time will tell