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.

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Original NASA ASRS Text

Title: 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.

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.

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.