Several years ago when I was doing rent to own properties, I had a tenant who took wonderful care of our property. He even went so far as to add central heat & air and several other upgrades. For the first few years, I inspected his property regularly but over the last couple of years, I stopped doing inspections because he had so much money invested in the property and I had not ever had any issues with him. What I didn't know was that he had become an alcoholic and he died in the house. Once I was allowed back into the home by the police, I noticed a really bad odor even after my cleaners had emptied and cleaned the house. All of us were stumped; I knew I couldn't put it on the market for sale until we discovered the source of the smell.
Summer was just heating up and one hot afternoon I walked into the house and noticed the ceiling fans were off and I just happened to look at the trap door to the attic and notice dark stains that looked damp. My painter brought in a ladder and as he lifted the trap door stuff started dripping. It was a nightmare over the next few weeks with the health department, workers, and the bats. Each time we thought they were gone and the attic was clean, we would come in the next morning and there would be a little group there.
How did we finally get rid of them? I paid for respirators and special zip up overalls and gloves. The bat guano was about three feet deep in two places and scattered throughout the attic. Insulation had to be pulled out. We kicked out the vent but health department had to approved disposal. Once it was cleaned out which had to be done at night when they were gone. We did every old wives remedies plus health department suggestions: sprayed gallons of winter green alcohol after scrubbing everything with clorox, filled every hole we could find with spray foam, covered all areas with door screens, put flood lights up there 24 hrs. a day for a week, put plastic owls and snakes all over everywhere, put in new vents and covered those with screen material.
Eventually, we were rid of all of them and the health department was satisfied but it took us all summer before it was over. By the way, the bat boxes didn't work either and we weren't allowed to kill the bats.