What had someone put in Clay's locker days before Hannah took the pills?

• a map of the city marked with red stars

A few weeks ago, just days before Hannah took the pills, someone slipped an envelope through the vent of my locker. The outside of the envelope said: SAVE THIS—YOU’LL NEED IT in red felt-tip marker. Inside was a folded up map of the city. About a dozen red stars marked different areas around town.