Jim decided to hold off and let Judy look around for a while. He did ever so slowly stick his head back out from under the pillow, and saw that Judy wasn't even looking in his direction. He watched as his giant sister called out to him a few more times, clearly confused as to where he must have gone. After a few moments, she walked out of the room, still none the wiser. Jim was exhilarated. She was so huge! So loud! And she was just nine! Being shrunk was fantastic.
With his secret intact, Jim took a second to take stock of the situation. Although he liked being tiny, he wasn't sure how much more he could get out of it in his current situation. Sure, he could explore the house as a tiny person and see what every room was like. But otherwise, it was just him and his nine-year-old sister in the house until his parents got home. What was the fun in that? He was supposed to be watching her anyway. No point in going missing before he'd even had the chance to have any real fun with this.
Of course, he really did like being small...