What was found
After landing and parking at the gate the right lavatory smoke detector went into alarm. smoke was reported to be visible in the aft cabin. All passengers were evacuated safely and CFR did respond. No source of the smoke was found and no evidence of any type of fire was located. The F/O did discharge a portable fire extinguisher into the lavatory. Maintenance cleaned the extinguisher agent from the lav, replaced the fire bottle and replaced the water separator bags. Aircraft has returned to service.