That was the good surprise.
We have a wonderful piece of her art on the wall above our fireplace and had lately noticed what we thought was some charcoal dust around the base of the matting.
She said that's fairly common and while we were discussing it, she asked to take the painting off the wall. Doing so revealed another surprise, and not a pleasant one.
Nasty black mold.
The paper on the back of the frame is damaged, so that black stuff on the matte may well be mold and not charcoal. We will be needing to head out to a frame shop and get this re-matted and I'm just very happy we caught this before more damage was done.
And we need to figure out where this moisture is coming from and correct that problem. Ugghh. More joys of owning a home.