There's no definitive answer to this question as cats react differently to different stimuli. Some cats may appear more alert and attentive when someone new enters their home, while others may seem uninterested. It's possible that cats can sense an intruder through their keen senses of smell and hearing. They may also pick up on visual cues, such as body language and movement patterns. If your cat appears to be on high alert when someone new comes over, it's possible that they're trying to protect you from a perceived threat.