B757 Captain is informed of smoke coming from the aft lavatory during boarding. The First Officer is sent back but the source is not apparent so Maintenance and the Fire Department are called. Maintenance discovers a door warning circuit breaker tripped and when reset the source of the smoke is traced to the aft door warning switch.

Narrative

While boarding for departure my aft Flight Attendant called the Purser and myself to report smoke coming from the aft lavatory forward of the door. She said she was taking a fire extinguisher and going in. I sent my First Officer to go back and assist and assess the situation. He told me that there were no flames and pulled the waste container out to ensure no items were burnt and halon extinguisher had not fired off. He also told me there was definitely smoke and it smelled electrical. I notified Maintenance and requested the Fire Department be notified to prevent this from escalating. The Mechanic arrived first and could not find the source of smoke so the Fire Department arrived with an infrared sensor and they decided to go ahead with an orderly evacuation of the aircraft as they also could not be certain of the source. Our mechanics sent me to the cockpit to look for popped Circuit Breakers (CBs). I didn't find any. However a mechanic went into the E and E compartment and found a door warning CB popped. When he reset the breaker; flames shot out of the wall near the door lock. (The same place the aft Flight Attendant had seen the smoke). With everyone now convinced of the source of the fire/smoke; Maintenance proceeded to change the door switch and we reboarded the aircraft and departed.