A lab
There was a lab beyond the door! He even thought he saw his book on the table! He hadn’t been able to look out the window for long since he was frightened of being seen. But he finally realized he was on the correct track.
Checking the room
He resolved to look through that window at least one more. He needed to know more about what was going on in that room if he wanted to get his book back. So he took a long breath and sat back down on the stool, thinking no one would notice.