With no familiarity of the house, Biff looked around to decide where to look first.
Naturally he ended up drawn to the kitchen, where nobody would wake up and where the only truly bright source of light in the house was: it was the microwave and the oven clocks, each proclaiming the time in big bright green numbers. One read 4:00 AM, the other read 4:02 AM. "Fix your clocks, Madison, one of them's running fast," Biff thought, not daring to speak aloud and wake up a house resident.
Nothing of investigative value was in the kitchen, but Biff thought to steal one of the knives anyway. Just in case; his bat obviously wasn't enough last time.
That done, Biff turned to leave. What he really wanted to find was which of the bedrooms belonged to Jon, but without a working knowledge of the house, Biff suddenly found himself a little daunted by the idea of opening a random door and waking up whomever was inside. Nevertheless, he creeped through the house as quietly as he could, and started looking about for Jon's room.