I had a couple invade my apartment’s walls once. A couple weeks straight just random scurrying around at all hours of the day. Then one died back there. Then the flies came. After another short amount of time my whole house was infested with flies. I killed a lot of them in whatever way I could but they kept coming. Finally I found where they were getting in through the tub surround and sealed it up and the problem ended. But oh, my, god I have never felt the same about squirrels since. And to dispel the question that may be in some people’s heads about how I could let this happen, I was renting, it was the landlord’s fault they got in in the first place, and he was a slumlord. Also, squirrels are just invasive assholes especially in areas with winter climates.