A bat visits

I had a bat in the room at 3 am. I heard a fluttering of wings by my
head repeatedly. Thought it was a moth. Eventually I see it, flying
just like in the Abbot and Costello movie with Dracula. I put the cat
in the closet and closed the windows. (All have screens on them.) I
then waited by the open front door as that was the only fresh air
coming in, which is what the bat was going for. In a little while,
the bat came to the front door. It was hard to see but I saw it crawl
around the screen. I opened the door. I wasn't sure but I thought it
flew out. I closed the door and opened the windows again. What an
adventure. How did it get in? From the screen by the kitchen vent?

Furnace causes this?

P8080009, originally uploaded by Lipwak.

Yesterday I filled the birdbath with clean water. Last night it rained. Today, as often happens, the birdbath is full of this dirt.

I used to think it was from whatever animals were using it (raccoons, woodchucks, who knows what). Now I think it is soot from my furnace. The furnace would have run during the rain, causing the soot to not get far. (The furnace really smells when it runs. It ought to be replaced.)