If the right side indicators are working then the flasher unit is OK.
Look for an open circuit (broken wire or dickie conector block, switch) in the left indicator circuit,,, haven't been working on switch block on handle bars by any chance ?
You mention bike has a security system,,,, is the left side still coming on when security system activates and de-activates?
have you checked under rear guard , had that problem ,the rear tire scrubbed the wireing out