Narrative:

After the main cabin (mc) beverage service and pick up was complete FA4 (flight attendant) called me (FA1) and notified me that [a] passenger (passenger) did not return to the aircraft after she deplaned during boarding. She had deplaned to retrieve her uncle's medication from the security check point (located in main terminal). We departed; requiring a tram ride from main terminal; where security is located. I was completely unaware a passenger had deplaned during boarding--I was busy doing catering checks; prepping galley; loading ovens; pre-departure beverages; making PA's etc. FA3 was occupied hanging coats and serving the drinks. Boarding was going on this entire time. FA4 stated she believed FA3 and I were aware a passenger had deplaned because boarding was held up briefly for passenger to come forward through first class to deplane. I was not aware of this; FA2 stated he thought the passenger was coming forward to give a bag to the agent for checking. He stated as the passenger rounded the closet with her bag he proceeded back down the aisle to perform his duties so did not actually see the passenger deplane. Apparently there was an agent at the door but I had zero awareness of any of these goings on. I was task saturated. FA4 states she told passenger to take her carryon baggage and tell the agent what she was doing. Upon my awareness of the incident I spoke to the remainder of her traveling party. My conversation with family confirmed [the] passenger had deplaned with carry on; she had checked luggage; she did not return to aircraft; she returned to main terminal security check point to retrieve [her uncle's] medication kit bag; and she did not make it back to the plane. Family had been in text contact with her and their main concern was how would she get out of ZZZ; what flight would she be on? When asked why they did not notify the cabin crew that passenger had not returned before we left the gate [the uncle] said essentially 'we knew the plane would wait anyway.' I said that may or may not be the case depending on where she was in the process; but we really needed to account for her; for security sake and for her sake and for your peace of mind. They were very kind and calm about the incident; their only concern was how/when [she] would get to [destination].I then briefed the captain with all I knew of the incident. He was able to contact dispatch and confirm that the passenger was #1 on the stand by list for [another] flight; with that flight showing 4 open seats at the time. I informed [the uncle]. They continued to text and make positive contact at various times; to include upon landing.besides the obvious breach in procedure; potential security concerns; and anxiety for the family-the captain and I became concerned about how many souls we actually had on board. At door close the agent first said 149. Then another agent came back down and said 151; but didn't explain how/why it changed. Then door closed. So now...with passenger deplaning and not returning...how many souls on board did we really have?in retrospect I recalled an agent stating something to the effect of all the passengers having made it 'except for that one.' I don't know if that is in reference to [the deplaning passenger] or another revenue passenger who didn't make the flight. A 'time-out' to ensure all fas are aware of what is happening- especially FA1 who is ultimately responsible for ensuring procedure is properly followed. Boarding should have been stopped- FA4 should have escorted the passenger and handed her off to me; or directly to the agent who was standing by; but definitely brief me on the situation. Communication. Slow down. Be deliberate. I feel awful about this event- I feel bad for the family. I am terrified that I let this happen on my watch for the security implication of such an incident. Thankful it had an innocent outcome; but I realize how serious it could be. I have learned a great deal and will include this in my future preflight briefings with my crews. Also; if the plane had crashed...how many pairs of shoes are we looking for? That scares me too. I don't know as anyone really knows and am beyond thankful that did not happen. Ask the captain--I was shaken to the core about this for those very reasons and the implications therein. The responsibility weighs heavy and I accept it completely. I feel terrible for what happened and am thankful there were no worse outcomes than some worry and inconvenience for the family. Lesson learned. Thank you.

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Original NASA ASRS Text

Title: An Air carrier Flight Attendant reported that she was notified by another Flight Attendant that a passenger did not return to the aircraft after she deplaned during boarding.

Narrative: After the Main Cabin (MC) beverage service and pick up was complete FA4 (Flight Attendant) called me (FA1) and notified me that [a] Passenger (PAX) did not return to the aircraft after she deplaned during boarding. She had deplaned to retrieve her uncle's medication from the security check point (located in main terminal). We departed; requiring a tram ride from main terminal; where security is located. I was completely unaware a passenger had deplaned during boarding--I was busy doing catering checks; prepping galley; loading ovens; pre-departure beverages; making PA's etc. FA3 was occupied hanging coats and serving the drinks. Boarding was going on this entire time. FA4 stated she believed FA3 and I were aware a PAX had deplaned because boarding was held up briefly for PAX to come forward through First Class to deplane. I was not aware of this; FA2 stated he thought the passenger was coming forward to give a bag to the agent for checking. He stated as the PAX rounded the closet with her bag he proceeded back down the aisle to perform his duties so did not actually see the PAX deplane. Apparently there was an agent at the door but I had zero awareness of any of these goings on. I was task saturated. FA4 states she told PAX to take her carryon baggage and tell the agent what she was doing. Upon my awareness of the incident I spoke to the remainder of her traveling party. My conversation with family confirmed [the] PAX had deplaned with carry on; she had checked luggage; she did not return to aircraft; she returned to main terminal security check point to retrieve [her uncle's] medication kit bag; and she did not make it back to the plane. Family had been in text contact with her and their main concern was how would she get out of ZZZ; what flight would she be on? When asked why they did not notify the cabin crew that PAX had not returned before we left the gate [the uncle] said essentially 'we knew the plane would wait anyway.' I said that may or may not be the case depending on where she was in the process; but we really needed to account for her; for security sake and for her sake and for your peace of mind. They were very kind and calm about the incident; their only concern was how/when [she] would get to [destination].I then briefed the Captain with all I knew of the incident. He was able to contact dispatch and confirm that the PAX was #1 on the Stand By list for [another] flight; with that flight showing 4 open seats at the time. I informed [the uncle]. They continued to text and make positive contact at various times; to include upon landing.Besides the obvious breach in procedure; potential security concerns; and anxiety for the family-the Captain and I became concerned about how many souls we actually had on board. At door close the agent first said 149. Then another agent came back down and said 151; but didn't explain how/why it changed. Then door closed. So now...with PAX deplaning and NOT returning...how many Souls On Board did we really have?In retrospect I recalled an agent stating something to the effect of all the passengers having made it 'except for that one.' I don't know if that is in reference to [the deplaning PAX] or another revenue passenger who didn't make the flight. A 'time-out' to ensure all FAs are aware of what is happening- especially FA1 who is ultimately responsible for ensuring procedure is properly followed. Boarding should have been stopped- FA4 should have escorted the PAX and handed her off to me; or directly to the agent who was standing by; but definitely brief me on the situation. Communication. Slow down. Be deliberate. I feel awful about this event- I feel bad for the family. I am terrified that I let this happen on my watch for the security implication of such an incident. Thankful it had an innocent outcome; but I realize how serious it could be. I have learned a great deal and will include this in my future preflight briefings with my crews. Also; if the plane had crashed...how many pairs of shoes ARE we looking for? That scares me too. I don't know as anyone really knows and am beyond thankful that did not happen. Ask the captain--I was shaken to the core about this for those very reasons and the implications therein. The responsibility weighs heavy and I accept it completely. I feel terrible for what happened and am thankful there were no worse outcomes than some worry and inconvenience for the family. Lesson learned. Thank you.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.