Clients house has been getting these stains every spring for 3 yrs. I clean them.off, and gone until the following spring. I thought bat, but when up on the ladder, there is no spot for a bat to get into. Believe me, I've dealt with bats, and know how small of a sliver they need to hide. Just can't figure out what critter or bug is making these spots.
Possibly in the bee, or wasp family. Tell your client to keep an eye out on it next spring daily.
