They said the car was parked at the front entrance of her apartment building at 41 Cowie Hill Road around 2:30 p.m. on Feb. 13.
That was the last day the Saint Mary's University student was seen.
"If a taxi was there, that person could be a possible witness," Sgt. Nancy Rudback said.
She said officers don't have any details about the make, model or colour of the car or even what company the driver works for.
Anyone with information about the taxi is asked to contact Halifax Regional Police at 490-5016. Anonymous tips can be sent to Crime Stoppers at 1-800-222-8477.