Our front-load washing machine has a problem where sometimes it really smells of sewer gas. This only happens occasionally, and we can only smell it when the door is open. We usually leave the door open so that it can dry out and not get moldy.
Presumably this happens because a trap is somehow losing water (maybe through evaporation?). There's a nearby bathroom sink that also has the same problem but I can just run water for a couple seconds to refill the trap. I can't do that with a washing machine. The washer and dryer are also installed in a narrow closet so I don't have easy access to the drain plumbing to see what's going on. Here's a photo of what I can see. That looks like hot and cold supply, and I'm fairly certain there's a drain going out the base on the back as well.
Any idea what the likely cause of the problem is and what it would take to fix it? I want to go in with a plan before attempting to pull the appliances out.
Edit: As a reference point, here's what's under the sink with the same problem.