What does the presence of the cat and the condition of the joad house tell Tom?

"A lean gray cat came sneaking out of the barn and crept through the cotton plants to the end of the porch." Tom figures,

By God! Look who's here. Somebody stayed." He put out his hand, but the cat leaped away out of reach and sat down and licked the pads of its lifted paw. Joad looked at it, and his face was puzzled. "I know what's the matter," he cried. "That cat jus' made me figger what's wrong."

Tom figures there aren't any neighbours or that cat would have left the empty house.