Psychic

How do they know? FancyFeast is more or less every 3rd night, but it's not set in stone. And yet every time the human intends to give them FF, they know. Before the saucers rattle. Before she goes to the landing where the FF is stored. They know. And the pitiful yowling, the hysterical milling about in the kitchen - which in fact interferes with the efficient delivery of FF - well, you'd have to see it to believe it. The human may attempt a video at some point in time, but they'd probably eat her. And the camera.