Our cat, Chin, can spot spiders on our ceiling. She will sit and meow loudly at it until one of us comes in the room. She's noticed so many it's gotten to the point that whenever we see her staring at the ceiling we look up paranoid there's a spider. About half the time there actually is.
She did this last night and noticed a particularly huge and disgusting one. I think she was especially loud because it was so enormous. It was promptly caught and squished outside. Thanks, Chin.