I completely understand anyone who dosnt believe in that. Im not christian, i have very strong views about christian god and id be outright banned immediately if i said them. I had an incident about 25yrs ago that proved to me spirits, ghosts are real. With milly and jesse there eyes get huge and trance like. Very very different to how jesse reacts to bugs or rats in walls. Once in the kitchen milly was on bench and was freaking and watching something, jesse stood at doorframe watching but was scared to come in. I politely asked spirit to leave and immediately milly head started going rnd and rnd like and orb was going rnd in circles in front of her. Then immediately both jesse and her were back to normal.I don't believe in that kind of stuff. (am an atheist). I think what the cats and dogs are reacting to are things we can't hear or smell...or maybe can't see - such as mice in the walls, or some bug flitting through air....but not spirits.
My dog AU, use to say on the sofa and was obviously looking at something behind my recliner... I would look and look and just not see what he was looking at...until one evening, I saw what it was...a very very small, short cobweb string that was being gently moved upon occasion by the heat of the torchier lamp behind my recliner. I couldn't see if from one angle, but in moving and looking a little more from his angle, I could see it.
In my 57 years, almost all the time, I have figure out what the animals were looking at...or hearing. I had my pets look towards a wall and a little later, I could then hear what they heard, a mouse moving around in it. Also, I think noises from outside penetrate into a home...noise our human ears can't hear. Again...found this out by seeing pets react to something...full attention at some place in the house where there was nothing happening...then I go outside and from that area of the house, I could hear a car horn way in the distance, or some distant neighbor playing music really loud.
I won't deny that it is freaky sometimes when pets do that...but I will always feel like it's not anything supernatural and just common stuff that they sense and we could too if our smell, hearing and eye sight was as sensitive as theirs are.