Date: 202006
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Aircraft: B737-900
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Airspace: Class E ZZZ
ATC / advisory: UNICOM ZZZ
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Training / Qualification
Second reporter function: Captain
Anomaly: Deviation / Discrepancy - Procedural Security; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: Pre-flight
Contributing factors: Company Policy; Human Factors
Primary problem: Human Factors

Narrative: During preflight; the First Officer; was the first one that entered the cockpit.  He started his normal routine; when he noticed an empty beer can in the garbage bag located behind the center pedestal in the cockpit.  We immediately removed the garbage bag from the flight deck; realizing that it was in violation of the FOM alcoholic beverage container policy. Initially; I thought a cleaner must have found the can in the back of the plane; and without thinking; saw the garbage bag in the flight deck and tossed it in there.  The flight attendants heard our conversation; and saw the beer can as well.  We all thought it was very odd; but didn't think it posed security risk; or threat; and I never thought for a second it was from one of my crew-members.  I placed the bag in the jetway; and carried on with my preflight duties; aiming for an on time departure. Once airborne and after further discussion with the FO (First Officer); we decided there are any number of scenarios for how the beer can got there. The previous day; we had flown that same airplane.  We had an open write up for an inoperative lav waste indication system.  We had messaged MX (Maintenance) about it; and as we were walking off the plane; we discussed it with the two gate agents on the jetway.  They informed us that Contract Maintenance would have to be called out. I also recall seeing two cleaners come on board; I believe one male; and female as we got off the plane that night. The next day; after removing the garbage bag; I reviewed the MX log; and noted that maintenance had placed the MEL (Minimum Equipment List) on the plane for the lav waste indication system.  So; Maintenance had clearly been on board at some point between the time we blocked-in the previous night; until that next morning when we reported for work.  Again; I am not sure how the beer can made its way into the flight deck garbage bag; but we might want to take a look at our security procedures.  Also; it should be noted that it was a beer; which the flight attendants explained; we sell onboard our flights.  All information is in the event narrative. Remind employees of the FOM policy for alcoholic beverages beyond the cockpit door threshold.  Review security procedures; and make sure there is someone over seeing Contract Maintenance when they are on board.


Second narrative: No Additional Information Provided.


Synopsis: Flight Crew flying 737-900 aircraft found empty beer can in cockpit during pre-flight.