Date: 201907
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Time Pressure; Communication Breakdown; Situational Awareness
Anomaly: Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Passenger
When detected: Taxi
Result: General None Reported / Taken
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: After flight and after gate arrival at ZZZ; a male passenger stopped by the cockpit on exiting and introduced himself; advised he was involved in the safety field and alleged that we had taken off with a father and small boy in the lavatory. He further alleged that I knew about this prior to takeoff. I responded that I was not aware of any such issue; and would not have departed had I known. He said he had cell phone video and observed the aft FA (Flight Attendant) make a interphone call to the cockpit. Noting this was a serious safety issue. I concurred with him that if this occurred it was indeed a safety issue and that I would investigate and report as required; but I had not received an interphone call from the FA. He reiterated the cell phone video and stated his intent to submit it to [the company] and the FAA. He provided no further details; nor showed me the video; and departed. As background; the flight was operating about 5 hours late due to aircraft non-availability. Once we had an aircraft; we boarded and waited another 25 mins or so for a replacement FO (First Officer) as the original FO was now projected illegal. The new FO was a [distantly-based] new hire; but this had little if any impact on the flight. On taxi out we stopped for about 20 minutes in the west hard stand to receive a complete weather reroute from ATC; reentered the new route; got approval from dispatch to operate on the new route; changed runways and departures; briefed and accomplished flows and checklist. We had 4 minutes to make our [Departure Time]. I immediately made the 'FA prepare for Takeoff PA'; cleared to line up and wait by ATC; cycled the seat belt chime to alert FAs we were taking the runway; sat for 15-20 seconds and were cleared for takeoff. Uneventful flight to ZZZ till passenger approached me (FO was already gone) at gate in ZZZ. All of this delay information just indicates that passengers; especially a small child; might well have needed to use the lavatory. So I did query the Flt Attendants about this incident. The aft FA did make an interphone call to the #1 FA; following his chain of command; to advise that a father had taken his son into the restroom. The #1 FA told me he did not contact us on the Flt Deck because we had 'already started rolling for takeoff' and he was concerned about violating the sterile cockpit in a critical phase of flight. He made a quick decision that overall safety was better served by not contacting us. Clearly taking off with anyone in the lavatory is unsafe and against [company] SOPs. Unclear to me in this event is exactly when the father and child entered the lavatory. If during the hold short or line up and wait (15-20 secs) phase; a call could be appropriate to the cockpit to alert us of the situation. We could decline the takeoff clearance and clear the runway as needed. If in fact we were accelerating (rolling?) for takeoff; the passengers' safety may be more compromised by the rapid deceleration of a rejected takeoff. The FA's situational awareness is limited to our departure PA; and taking the runway chime followed by aircraft movement and engine noise. This limited input could affect their decision making process as to where we are in the takeoff maneuver.Secondly; I'm concerned about communication. I brief the #1 FA on every flight; I believe this to be important since we now fly with many different crews; from many different bases; frequently different on every leg. I did so on this flight to relay details of the flight and to open up communication. Still no one called to advise us of this event during the entire flight. I only learned about it from the concerned passenger at the arrival gate. Kind of blindsided. Not even the father in the lavatory said anything. Of note there were 10 uniformed crew members on the flight who said nothing when departing; though they may not have been in a position to observe anything. Bottom line is passenger safety is paramount; I believeunless we're on high speed takeoff roll; we could have handled a reject safely if notified. Perhaps a more assertive approach by the FA would have prevented this event from progressing to a takeoff with passengers not in their seats.




Synopsis: A321 Captain reported being informed after landing of passenger using lavatory during previous takeoff roll.