Had the same thing happened to me. Bought a house that the previous owners were chain smokers. Spent all day getting the kitchen painted a nice brick red. Left to go get supper. Came back to the same beige walls from before painting. All of the paint had slipped down and off the walls. Great mess to clean up and start all over.
IF they also had a hobby of reloading bullets, that fine black powder might be easy to clean up with just one match… of course, it will just make a bigger mess to clean up.