One of my cats, Sharky, a 6 year old male, has this weird habit of going into empty rooms and "yelling". It's his "I'm lonely!" call, like a trill/meow. He'll do this until my husband and I either call him to us or we go to him to snap him out of it, and he always acts so happy to see us, like we've been away for a whole day. He's healthy otherwise and none of my other cats do this. Sometimes he'll even sit in the same room as us, facing away, and start calling out as if he's talking to ghosts (probably not). It's like he forget's we're there with him and again, he acts really happy to see us when we snap him out of it. What the heck is this behaviour? Any ideas?