I believe that this question is asking about a brief bit of narration that happens in chapter 5 which is titled "The Evil God and the Secret Spy." The chapter begins with April, Marshall, and Melanie returning to the abandoned lot, and they quickly figure out that everything is exactly the way that they left it previously. April and Melanie then begin clearing out weeds while Marshall stands guard in the alley.
Once that is completed, they head over to the "Temple" which is actually a wooden platform about 12 inches off of the ground. The two girls sit down to rest for a minute on the Temple's floor, and April notices a door on the opposite side of the lot. It is the only actual door that leads to the area, and the girls notice that it is locked with a padlock from the outside. This locked door reminds the girls that the Professor has access to this lot. This worries Melanie, and she asks what the Professor might do if he caught them there.
April doesn't believe that the Professor would mind as long as they don't bother him or hurt anything. April also is quite confident from the previous mess that nobody has been in the lot for quite some time. Melanie isn't quite so confident, and she suggests that maybe the Professor spies on them through the overlooking window. They check out the window and discover that it is so dirty that nobody could look through it even if they wanted to.