Andrew had been about to reply to Spike when he heard the voice as well. "Who's that?" he whispered just before he ended up with a lap chock full of vampire. But Andrew was afraid, and if Spike was in his lap then whoever it was outside of the door would have to go for him first, because he was sort of shielding Andrew with that position.
Eyes wide and whimpering, Andrew wrapped his arms around Spike's waist tightly, pretty much clinging to him. He didn't want a murderer coming in and, well, murdering them. That would be really really bad. And even if it wasn't a murderer, there was all sorts of bad things that could happen thanks to being around people that you don't know. Like... They could be sick! He could get sick from them! Spike wouldn't, but he would.
Andrew was itchy. What if this mystery person that was out in the room had brought in a disease and it was already effecting him? What if it was something that started with itching but led to death!? Oh no. This was not good. So Andrew just continued to cling to Spike, hoping the person went away and took their deadly itchy disease with them.