Police have issued a photograph of the hire car driven by a 26-year-old woman who disappeared before Christmas.
Gemma Garn, from St Dials, Cwmbran, Torfaen had been out with friends in Pontnewydd and was last seen during the early hours of Saturday, 19 December.
Detectives hope the picture of the black Kia Rio car, which was found on the Second Severn Crossing, may jog people's memories.
Anyone with information is asked to contact Gwent Police on 101.
A Gwent Police spokesperson said: "We are trying to target people who used the bridge to see if anyone remembers the car or seeing her [Ms Garn].
"We're also trying to piece together her journey from Pontnewydd, where she was out with friends, to the bridge.
"Someone may have seen her car, or perhaps saw her or served her in a shop en route from Pontnewydd."
The hire car was found on the bridge at about 1150 GMT on 19 December, about six hours after it was believed to have been left there.
Police and Ms Garn's family said at the time they were concerned for her safety because she had never gone missing before.
Ms Garn - who graduated from the University of Wales Institute, Cardiff in 2006 - is white, about 5ft 6in, with short dark hair with blonde highlights, blue/green eyes and a fair complexion.
She was last known to be wearing a black polo neck top underneath a black and white patterned dress, a brown casual jacket, black tights and black high-heeled shoes.