* raids come days after terror threat level raised to 'high' * Australia worried about militants returning from Syria, Iraq * Plan involved snatching member of public and beheading - ABC (Recasts with reports of beheading plan, adds Abbott comments) By Matt Siegel SYDNEY, Sept 18 (Reuters) - Australian police will allege that members of a group targeted in a sweeping counter-terrorism operation on Thursday planned to behead a random member of the public after draping the victim in the flag of Islamic State militants, Australia media reported. ...