“Items detected with you”

Yesterday my girlfriend took a train to come visit, and received a message “Items detected with you” on her phone. It was first detected at an early point in the day but the message arrived later once she got off the train. When you click on the message an unidentified AirTag is the culprit, and at the bottom it says her location can be tracked. We have found no sign of the AirTag and the location mapping shows it never left the station after we left the area. We turned off her location services and FindMy to stop sharing her location. I’ve searched everywhere and found no answer as to how to get rid of this without the physical AirTag. Is this possible?