(Portland, OR) -- Police are seeking the public's help in their search for a missing woman. The Washington County Sheriff's Office says 23-year-old Demi Helenius was last seen Saturday night at the Landmark Worldwide Forum at the Embassy Suites Hotel near the Portland Airport. Her parents say she didn't return home Saturday night, and officials say she didn't attend the conference yesterday. Police say an unidentified man gave the woman's cell phone to a Dairy Queen worker in Creswell as he passed through the drive-through Saturday night. There's no word on how the man got her phone. Anyone with information is urged to contact police.