Bat colony in attic
A pile of bats droppings in the corner of an attic
The end of a metal ridge vent. A favorite entry point for bats and flying squirrels into an attic, if the plug in the end is missing.
Typical stain on a louver resulting from the oils in the bat's fur as they pass in and out of the louver. Similar stains can usually be seen wherever bats are entering and exiting a building.