He misses you. Lol! He might have a separation anxiety. My dog had that but we started spending more time with her and gave her a lot of love when we seen her and she all of the sudden didn't have it. I don't really know how to treat it for howling because my dog just chewed stuff up and ate it.
Some cats are just "talky". My mom had this very sweet tom who cried every time she went out of his sight. At night when she went to bed he would sit at the bottom of the stairs and cry and it sounded for all the world like he was saying "Mowm?, mowm?"