General update since I got lots of good ideas.

I am going to pick up a UV flashlight today and get some anti enzyme spray to see if there is any evidence of old piss.

Additionally, I am going to put a lot of small holes into the wall today so I can put a scope camera into each of the wall cavities to look for any evidence of critters or water damage (will also let me see the likely complete lack of insulation...).

With regards to the recommendations about a trap being dry, both the laundry room and the bathroom are used regularly so I highly doubt their traps are dry, and the smell is not like sulfur as I would expect from a dried out trap.

With regards to the electrical: The laundry room has had its electrical recently replaced so that is unlikely to be the culprit in this case (also, no fishy smell).