Something Strange About Marci is the fourth story in More & More Tales to Give You Goosebumps.
Plot
The narrator (never named) wonders why Marci, a girl who's new in town, has suddenly arrived from nowhere with no family. Marci frequently shows up to watch the narrator and his friends play, but doesn't try to approach them, and leaves if they come near her. The narrator observes that she carries a briefcase with a mysterious object inside. He becomes suspicious of Marci and vows to find out everything he can about her.
The narrator eventually follows Marci to her home, which turns out to be a tent. She finally speaks to him, and offers him some fruit. When she begins to speak into the mysterious object (a recording device) the narrator finally realizes that she is a human scientist — unlike him and his friends, who are all orangutans.