On the Palm Beach Daily news website they have a really great shot of the fire and hazmat trucks coming down Sunset Avenue to investigate:
Caption: Two arrows made of white powder were left on the beach near Sunset Avenue. Hazmat was called to investigate Thursday morning. Near the arrows were pieces of a paper bag for Publix All-Purpose Flour. The arrows pointed to Sunset Avenue where more arrows and the letters "QF" led one street over to an "X" near Main Street and the post office.