My wife has been nicely bugging me about what she thought was an air conditioning leak: above our front door (exterior wall), we get a white residue build up. I crawled around in our attic and saw no leaks.
An air conditioning guy was at our neighbor's house and my wife asked him to come over to look at our "leak". He did, took one look and said that we are lucky - we have bats hanging out (literally) above our front door. He said that we should "Google for how to build a bat house" to encourage them to stick around (bats eat a lot of insects and are considered to be useful wild animals up here in the mountains were I live).
This also explains the mysterious tiny brown nobules to the left of our front door: bat droppings! They were so tiny that they just got cleaned up when we periodically clean up our entry way.