Akito's senses are heightened, yes, but with everything around him smelling and sounding unfamiliar it's difficult to pinpoint any threats. He feels safe enough in his pond that he doesn't keep his guard up anymore - not after the first couple of days. Clearly it wasn't long enough, because the sudden comment frightens him half to death; Akito abandons his strawberries and dives to the bottom of his pond, swimming partway into the den he dug for himself to keep most of his body hidden while still being able to peer up and wait for the robot to go away.