Forum Discussion
When I look at your photos, it helps to narrow down the possible issues. Based on what I see, it looks like picture number one reveals what was under the seat cushions, and picture number two shows mold on both sides of your hot water tank enclosure. BUT it also seems like there's mold higher up.
If that mold is indeed higher and your hot water tank was sealed in with the plywood AND cushions, I suspect water came from above. Is there any way to use a camera or cell phone to see what's behind that wall board?
If the mold was just under the seat, it could suggest a leak in one of the lines to the hot water tank, spraying water around. But if there's no mold on the floor under the seat or signs of water, like disturbed dust with a trail, and the mold is higher up, it likely came from above. This would prompt me to inspect the roof for any damage or missing parts, like a cap for a fridge vent.