Date: 202010
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Night
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class C ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Situational Awareness; Confusion; Distraction; Communication Breakdown
Second reporter function: Captain; Pilot Not Flying
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Airport; Environment - Non Weather Related; Human Factors; Procedure
Primary problem: Human Factors

Narrative: I was the pilot flying.  I had never been to ZZZ.  Captain was pilot monitoring.  During descent; ZZZ Approach advised that the tower would be closing soon and to advise ZZZ in sight for a visual to XX.  We had reviewed all applicable Jeppesen pages including Tower closed procedures.  ZZZ confused us initially with the statement Tower would be closing soon; as it was a little after XA00L [local time]; so we didn't know if that meant we would be getting in before they officially closed.  We got the field in sight and Approach cleared us for the visual and said to contact them on the ground to close our flight plan.  Focusing on flying; I don't remember them saying to switch to CTAF or XYZ.X for advisories; but I knew that was the plan and didn't see that when the Captain switched frequencies and began making call outs that he had switched to UNICOM instead of CTAF.  While I haven't done a lot of flying into airports with a closed Tower; when I have; it wasn't uncommon for transmissions in the blind to go unanswered so it didn't concern me when no one responded to our calls.  I followed the guidance to align with [Runway] XX and started descent on glide path.  It was right around 1;000 when I called stable that I noticed yellow flashing lights in the center of the touchdown zone.  I mentioned to the Captain and said I thought there might be a vehicle on the runway.  We continued and just after he called out the 500 foot call I noticed the light moving down the runway and then could make out brake lights illuminate on the vehicle.  I called out I was going around for a vehicle on the runway and the Captain acknowledged.  I executed the go-around and we went back to approach frequency and advised them what had happened.  He cleared us for another visual and we again made all the appropriate calls; although on the wrong frequency; and landed uneventfully.Although it was clear in the company pages; we had some initial confusion as to exactly when the Tower would close.  NOTAM had said XA00L; but it was after that; approximately XA05L when approach said Tower would be closing shortly.  We were only 5 minutes out so I think we both had some impression that Tower may still be open when we landed.  On the Jeppesen pages UNICOM is listed right next to Tower/CTAF and I think as things were happening quickly when Tower cleared us and we realized Tower was now closed we were configuring and I believe the Captain just looked and grabbed the wrong frequency.A more thorough discussion between both pilots to make sure we are both seeing the same information and are on the same page as to what the exact procedures we will follow in unusual operations such as a closed Tower.


Second narrative: Flying with ZZZ Approach vectoring for final to Runway XX when Approach warned of the Tower closing. First Officer flying and proceeded to the outer marker and called the field in sight. ZZZ Approach advised to switch frequency; but to call back and cancel IFR after we land. I looked at the airport diagram and saw UNICOM and used that frequency to announce our position. After we were fully configured and on short final we saw a vehicle on the runway. We continued until about 300 feet then did a go-around called ZZZ Approach to advise of our situation. He offered vectors; but we had the field in sight went back to UNICOM and landed canceled IFR clearance and taxied to the gate. First day back flying after taking extended time off. Leg 3 at night and not expecting the Tower closure. After reading the FOM the next day I realized CTAF is the correct frequency to use. Also looked at 10-9 pages and found the note about Tower closure procedures. Having been to an uncontrolled airport once in my career and the First Officer running the radio I was unaware of the procedures. Why would UNICOM frequency be listed?


Synopsis: Air carrier flight crew reported executing a go-around after seeing there was a vehicle on the runway. The Tower was closed at time of the event and the crew was on the wrong frequency.