Is it anywhere near something like a washing machine pipe? I'm guessing you'd have likely noticed if it happened when the washing machine was switched on but figured I'd ask in case.
Going back to working in an old building where there were pigeons getting in to spaces behind the wall, if I remember right, the smell was transient. It linked in with me and others getting bites (I ended up with a bad case of cellulitis from it), when it was investigated and the wall opened up, there was a dead bird (or more) and stagnant water and the smell was beyond vile.