Stink in Washroom - Possibilities?
Our second shower sits unused for weeks and sometimes months at a time (daughter moved out.)  About a year ago, a horrible smell started emanating from that bathroom - after many unsuccessful attempts to kill the smell, I finally realized that the P-trap under the shower floor must have dried out, allowing sewer gas to rise up through the drain.  I ran a couple of gallons of water, and the smell went away.  I run that shower for 30 seconds or so once a month, and the smell has never come back.

So: any unused plumbing fixtures in that washroom?