Start charging her for each visit when there are no visible issues and see how fast it stops!!!

We has a resident that had a kureig coffee machine next to the sink. She would over fill and the water to leak under the sink into the cabinet. She was saying the same thing it smell bad looked dirty and something was leaking. We had MANY different techs check it out until one took a video and we figured out the issue

Rap the stack and see if it absorbs the moisture.

It could potentially be something really simple like they are splashing water around, it's pooling around the base of the faucet, not getting dried and then simply slowly seeping through. That would explain why it's never replicated by maintenance because they aren't in there making a mess.

Have you gone there? Go with them and be truthful ..,we have reason to believe it is Xyz and how can we help…

We have had many water intrusions lately that are from a ‘y’ drain pipe that has cracked behind the walls; often times from apartments above but coming down all 3 floors and pooling at the ground level. They didn’t start happening until 2 years ago when we had an earthquake. Do you have moisture meter you can put on the back wall of the cabinet?

I’ve had 2nd & 3rd floors with the problem and the studs around where affected and the fact that is was Grey water added to the smell. Just a thought. Maybe a plumber would have a heat camera to see if they get an ‘iffy’ read behind the wall.