Foul Smell in 7 year old laundry room

we returned 5 days ago from a week of vacation to find a new (and foul) smell in our laundry room; the home is 7 yrs old & this has not happened B4. I can't claim to have a nose that can relialbly discern difference between sewer gas & a dead mouse but I'd lean slightly towards the latter. There is no floor drain. The tub sink is used often so a dry P-trap is unlikely; same for the "wall drain" that the washing machine uses (white plastic box that also has water line hookups)...the washing machine is used so much (3 children) that a dry P-trap is hard to imagine. Have been unable to localize the smell...holding nose over either sink or wall drain has not been productive, though I tried stuffing a rag in the wall drain and leaving for a while to see if it made a difference (didn't). There is a gas line (stubbed off) that penetrates the floor behind the dryer; the hole in floor is larger than the pipe (maybe enough for a mouse to get through) so I tried stuffing a rag into the space (maybe 1/4 to 1/2") around the pipe in case that was it, but can't tell a difference. I guess my next step will be to drill holes through the base of the drywall behind the washer/dryer to see if the odor becomes more intense, though I don't know how a mouse could get in there. Also considered unhooking the washer and tipping it over to see if a mouse could have crawled up into it from below and died. The washer and dryer are 18 years old for what it's worth (but they work OK). When we enter the room, the smell is obvious but once we're in there for a few seconds it become impossible to localize the smell.....driving my wife crazy! Any ideas?

Had a rat crawl into the dryer vent one time. This was a basement installation and once he fell down the vent the only place that he had to go was into the back of the dryer, where he died. Is it possible that a critter could have gotten in that way? It might be worth checking out.