Chow chow behavior (scratching/digging)?

I got myself a male chowchow that was given to me by a friend. He's a chow indeed with all the normal aggressive behavior and all. I am just puzzled lately with his behavior when he was scratching the marble tile where he's been spending time lately. It's as if he's trying to dig up something. He's not doing that in any other place even in the lawn or dirt in the backyard. Any thoughts on this odd behavior?

Also I'd like to ask if it's okay to give a dog multi-vitamins meant for humans. I had a friend who gave me too many vitamins and I just found out that its nearing it expiry date. Would it be okay if I mix some in my dog's food? Like half a day even?