I think cats are weird as a rule, although usually endearingly so. Dogs, as much as they can and will bark at almost anything, rarely actually get freaked out. When your dog starts
growling or whining for no apparent reason, or refuses to cross an arbitrary barrier in your home, then it's time to start getting suspicious, IMHO.
I don't necessarily doubt that there's an "additional family member" in the house, it's just that I wouldn't recommend using a cat as a litmus
.