One bit of information I discovered last year is that the p-trap can provide a breeding ground for fruit flies. I cover or close the drains on sinks I don't use often. I doubt, squirrels c an get through but I do know rats can get into the sewer drain and come up in the toilet.
??? I think you missed the point of the post?
@SkotlandSkye I know it was supposed to be a joke but I just thought I'd add a little 'fun' of my own, sorry.