Agree with the "in unison" thing. Two of my cats are related (mother & son). I've seen this a couple of times and yes, it's creepy as hell. Although like you I just think "I wonder what they're picking up on". Still, it kinda' sends a chill down your spine a little.
I keep meaning to do that Queen Mary ghost ship thing around Halloween time but either don't have the time or just flat-out forget about it each year. I'll try to remember this year. I ain't afraid to stay on there at Halloween - it'd probably be kinda' cool. For added measure of "supernatural activity detection" I suppose I could bring the cats along. . .
Ever watch "Ghost Hunters" on SciFi? That show is a crack-up. Those dudes take themselves way too seriously and jump to way too many conclusions under the guise of trying to be objective. It's quite funny.