Communication Breakdown; Troubleshooting
Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Passenger Misconduct; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Environment - Non Weather Related; Human Factors
As I was securing up the aft galley; the smoke detector started to sound. I knocked on the aft lavatory; as there was a passenger using it.  'Are you ok?' I asked.  ' Yes. But what's that sound?' She replied as a big plume of smoke/vapor came toward me when she opened the door.[I pointed out that] smoking on an aircraft is a federal offense. So you really don't want to do that. I checked the waste receptacle and the shelves above it. As well as the toilet bowl; sink; and the little cupboard that holds the toilet paper and Kleenex. All clear. It didn't smell like tobacco.I asked the passenger if it was an e-cigarette; she said yes. I let the CA know what happened so he could handle it on his end. The passenger seemed totally taken aback that smoking wasn't allowed on planes and kept repeating that she didn't know it wasn't allowed. Also wearing the personal; disposable face mask made it hard to smell the smoke/vapor plumage that came out of the bathroom. That's why I asked her if it was an e-cigarette. Since I didn't see any smoking device/paraphernalia.As helpful as the personal face masks are for filtering out bacteria and viruses; it's difficult to make out scents fully. But since it was an exhalation of vapor from an e-cigarette it's not like I want to take off the mask to be able to identify the smell due to COVID concerns. That's why a thorough search for any burning or foreign material was done along with asking what she was using.
Flight Attendant reported that the smoke detector alarm was triggered by a passenger smoking an e-cigarette in a lavatory. Flight Attendant stated that wearing a personal face mask made it hard to smell the smoke/vapor plumage to determine whether the smell was due to an e-cigarette or other burning material.